Hypothetical Spec

[VoX]

64-bit allows you to use all of your RAM, 32-bit limits you to just over 3GB. That is ofcouse if the application supports 64-bit, if not it is run in 32-bit and you simply only use 3GB of RAM on that App.

Check most games, some do include an ATI requirement, but not all. Games which one of them back is likely not to have a requirement for the other, although it will work fine.

PhysX is only nVidia.