I say go 64bit. I debated that same question for my new build - 32 or 64. Well, i went 64bit vista ultimate and i couldn't be happier. It is completely stable. I play COD4 and Crysis mostly and they both run flawlessly. All my peripherals work and i haven't had any trouble finding 64 bit drivers. However, if you have some older stuff (printers and the like) you may have some issues.
You can still run all your 32bit programs with out issue. In hind-sight, its really a no-brainer.