If you want to keep your options open then get a stem with a 31.8 mm clamp. That way you can use a wide variety of bars, ranging from 25.4 to 31.8... just get a shim for the smaller ones. I actually wish I'd thought of this when I got mine. The set of track drops I wanted only comes in 31.8 mm and won't fit my tiny 25.4 mm stem. :'(
Oh and remember to also buy check steer tube diameter. Would suck to get a new stem that fits your handlebar perfectly but is too tight or too wide for your steer tube. Usually it's either 1" or 1 1/8".