I have had good luck running Vista 64 with the games I play (LOTRO, WAR, L4D) and no driver issues.
If you have more than 4GB in your Mac, you'll definitely want the 64 bit version of Windows or you won't be able to see the memory. Even with only 4GB, 32 bit will only see ~3.25GB of that.
Most games don't need, nor can they take advantage of a 64 bit environment (a few rare exceptions like Crysis or Farcry (can't remember) are available in native 64bit). But, it doesn't seem to hurt to run 32 bit games in 64 bit from my experience.
If you install 64 bit, you can have both worlds. Install 32 and you are out of luck if you need something 64 bit.
Mike