Vista x64 is excellent compatibility wise, unlike XP x64 back in the day.
You'll be needing x64 Vista anyway if you wanna use over 3GB of Ram, there's no real drawbacks to using x64 either as most modern stuff is bound to have a x64 driver if it has a 32-Bit one for Vista.
Not sure if x64 is any faster than x86 but I've heard it is slightly ,you may as well, no real drawbacks like I siad.