As I recall, PS CS4 came with both 32 bit and 64 bit versions in the same pack and I installed both (but hardly ever use the 64 bit because of the plugins issue). But it's easy to check that with Adobe, there's probably a FAQ.

You could certainly upgrade your individual licenses, which might or might not be cheaper (I think not, but it might not be by much).

Yes, Adobe's licensing/bundling is Byzantine and you really have to read their site and check and re-check before parting with cash, IMO.

Steve