Most all modern day CPU's support both 32 bit and 64 bit OS's. If you will be using more than 4 gig of memory, you must use a 64 bit OS.
I recommend 64 bit anyhow unless you have older devices that don't have 64 bit drivers for them.
I just noticed you posted this in the XP section. I do not recommend XP x64 as it doesn't have good driver support. Vista and 7 have excellent 64 bit support.