Personally I would go for the 64 bit version if you are going for Vista.
Vista has some problems and those problems are the same for the 32 and 64 bit versions.
Virtually everything that works in Vista 32 will also work in Vista 64.
First going for a 32 bit version and later switching the a 64 bit version is twice the effort without any advantages.
Should you happen to have 32 bit version of XP handy then I would advice creating a dual boot system but installing everything in Vista 64 first and only using XP 32 for programs that simply do NOT work in Vista 64 (hopefully the upcoming SP1 will fix the remaining issues).
Performance wise you won't see any big differences. The 64 bit OS allows addressing more memory (even if you don't actually have that much memory) so in all (64 bit) 3D programs you can see a major increase in the complexity your computer can handle.

I do have a dual boot system and for the last 6 months I'm still waiting for the right moment to make the switch. Just reinstalling all software and setting up everything will take a week. All my critical software works but until I've installed everything I can't actually switch over and I just don't seem to find the time.