1) the one you bring.
I compete using Ruger 4" GP100 357 Mag, 5.5" Redhawks in 41 and 44 Mag, EAA Witnesses in 9x19, 9x21, 38 Super, 40 S&W, 41AE, and 45ACP.
You may also find me using a 1911 in 45ACP drawn from an IWB holster, stuffed with 8-rd magazines.
You can use any gun over 38 Special / 9x19, and you will compete in a class with shooters of similar skills and similar guns.
And it is, without question, really really fun. You will also learn improved gun-handling and safety skills.
www.uspsa.org (USPSA Home Page -- use 'club-finder')
www.macten.net/ipsc/gmps.html (GMPS - my home club page)
http://home.adelphia.net/~eboadway/ipsc.htm (GMPS clubmember picture/results page)