I've been lurking for a while and have been meaning to introduce myself in the appropriate area, but I figure this is as good of place as any since I have a little bit of info that may be of some help, although y'all did cover it pretty well.
First, my name is Bryant Chaffin, and I am the Mississippi Section Coordinator for USPSA competition. The title doesn't mean much, but it does mean that I talk to new shooters on occasion and serve as the liaison between Mississippi clubs and the National Organization. In the former role, I have put together some info about the different divisions that I often send to potential new shooters when I get questions about USPSA/IPSC matches. I've posted it below along with some updating and additional comments. Hope it helps. Also, if I can be of any assistance, either on this subject or otherwise, please feel free to send me a PM and I will be glad to try.
The short answer to the question is just about any safe handgun 9mm / .38 or larger can be shot in USPSA competition in one or possibly more divisions. Local clubs often let children compete with .22's, but the rules technically require 9mm / .38 and a minimum "power factor" (calculated as bullet weight in grains X velocity in fps / 1000 ) of 125. This is due to the "practical" part of the sport. The sport's motto is "D.V.C.", which stands for "Diligentia, Vis, Celeritas", or "accuracy, power and speed." All three are needed to do well, but the balance shifts from one to the other depending upon the stage being shot and its particular requirements.
There are 6 divisions: Open, Limited, Limited 10, Production, Revolver, and Singlestack. Each division is scored separately, so you are only competing against similar type equipment. Most clubs will do an "unofficial" overall that scores all divisions together, but this is just for comparison purposes and is not considered official match results.
"Open" is just that, basically unlimited. The guns are very specialized, high capacity with long magazines (limited only to 170mm in length, which yields up to 27-30 rounds!), compensators, and red dot sights. Handloaded .38 Super or 9mm is the dominant caliber. There are usually several at a match, but it is not typically the largest or best represented division, depending on what area of the State you shoot. Having just looked at the numbers for 2009, Open is officially a close 4th as far as numbers of shooters in Mississippi throughout the year, although interestingly, it tied for second largest at the Mississippi Classic State Championship Match last May. If anyone is interested in this division, I highly recommend that they shoot some of the guns first and talk to some of the people that shoot it. Race holsters (or minimalist competition inspired) are allowed and gun and magazine pouch positions are almost unlimited. As mentioned above, the guns are very specialized and this is typically the most expensive division. Used equipment can be found relatively inexpensively on occasion.
"Limited" is slightly more restricted. No compensators, red dots, or magazines over 140mm allowed. This division is dominated by STI, SV Infinity, Para Ordnance, Springfield Hi-cap, or Glock .40 or .45 caliber guns with basepads on the magazines giving capacities of 17-21 rounds. S&W M&P's or Springfield XD's could also be competitive, along with a few other choices. This is the largest division in the State throughout the year and bigger divisions at most matches (not all matches) and the largest at the MS State Championship by 50% or so. I must go ahead and admit my bias, as this is my personal favorite and what I have shot pretty much exclusively since I started in 2000. Race holsters are allowed here as well. There is a slight scoring disadvantage for anything smaller than .40 ("major", 165 power factor or above) (i.e. 9mm or "minor", 125-165 power factor), because of the theory that it is easier to shoot faster because of less recoil in the smaller caliber. Most major caliber guns are .40's. They hold an extra round or two over .45 in the same length magazine and are cheaper to shoot than either .45 or 10mm. Although custom guns can get expensive, they are not necessary to win, as Glocks and Paras with a few modifications and some basepads on the magazines have won local matches as well as State and National Championships. Used equipment also occasionally comes available for significant discounts off of new prices.
"Limited 10" is the exact same as Limited except magazine capacity is limited to 10 rounds, no matter what gun you are shooting. A smaller division at most matches, guns are the same as Limited along with Single Stack 1911's in .45 formerly being popular. Limited 10 is now the smallest division in the State as far as local turnout as well as the State match.
"Production" is just that, a "stock gun" division with very limited gun modifications allowed (generally changing sights, springs, internal trigger or accuracy work, adding grip tape, etc ok, but not much else), and the gun must have a double action or "safe action" first shot. Production is now the second largest division in the State and tied for second at the State match. This division is generally made up of Glocks, Springfield XD's, Berettas, Sig Sauers, CZ's, S&W M&P's, etc. There is an official list of acceptable production guns on the USPSA website. Click on "Match Rules" on the left side and then you should see the link to the list. Magazine capacity is limited to 10 rounds to even the playing field no matter what caliber, and no "race" holsters are allowed. It must be a typical kydex or leather hip holster. Magazines and gun must be placed behind the forward point of the hip bone (not on the same side of course). 9mm is most common due to inexpensive ammo and lower recoil and, importantly, all calibers are scored "minor" so there is no advantage to shooting a larger caliber. I don't deny this is one of the cheapest divisions to get into, although there are Production shooters that spend more on their guns and rigs than shooters in some of the more "racy" divisions.
"Revolver" is just that. Most shoot moonclipped .45's, with other calibers trailing by a good distance in popularity, and speed loader guns being very few and far between. Revolver narrowly edged Limited 10 state wide for fifth place last year in shooter turnout, but it was fourth at the State match. Reloading is, shall I say, frequent, so plenty of moon clips or speed loaders as well as a way to carry them is needed. Race holsters are allowed, but optic sights and compensators are not.
Finally, "Singlestack" requires a singlestack 1911 with .40's and .45's scored "major" and limited to 8 round magazines while .38 supers and 9mm's are scored "minor" and allowed 10 rounds magazines. Very limited modifications are allowed. No holes in the slide or other slide lightening, etc. Magwells are allowed as long as the gun with magazine inserted can fit in a box of certain dimensions. These dimensions can be found on the USPSA website. No race holsters. In fact, the holsters are more restrictive than Production, as the front strap of the handgun must ride above the belt. This is basically a "classic" 1911 division and, like Production, is one of the cheaper divisions to get into. However, also like Production, there will be some high dollar handguns used by some shooters.
In all divisions, while some gear may be more competitive than others, if the handgun is reasonably accurate and most importantly reliable, it is more often than not the "Indian" rather than the "arrow" that determines the outcome.
If you are shooting 8 or 10 round magazines, you need at least 5 magazines and pouches to carry 4 of them (1 will be in the gun when you start the stage). 4 total might get you by, but there is no room for error. 6 or more isn't ridiculous, although you will not have to have that many. With high caps, 3 is minimum but I recommend 4. When I say high capacity, if the gun holds less than 16 or 17, you will be at a disadvantage and might as well download to 10. This is because most target arrays are set up in 8 rounds or less. If you can't shoot 2 arrays before having to reload, you might as well reload while moving between each array, i.e. after 8 rounds or so, as loading in the middle of an array generally isn't fast.
The most important things if you come out to a match are to be safe and have fun. Your biggest competition is always your self, but if you can bring a friend or two it gives you some other "newbies" to compare scores against. Don't worry about where you place, especially right at first. You will improve. Most will do so from just shooting matches. Others will practice or get coaching. My recommendation is take it a little slow your first match or so until you are comfortable, as there will be some adjustment to the rules. Whether you are already a great shooter, just beginning, or anywhere in between, there is a learning curve as you adjust to a game you haven't played before. Ask questions. I have shot matches at clubs all over Mississippi and in 10 other states and there is always one thing in common, the vast majority of the people were very friendly and just great people in general.
I hope most of that made sense. If it didn't, please let me know and I will be happy to try and clarify. Also, if anyone sees me at a match, please say "hello." I know and have shot with a good many members, and I have met a few others recently, but I have also been at matches where other members were present and didn't have a clue until afterwards.
Have a good one,