For OP the choice for gaming is to optimize system for their favorites. They haven't given us a list, but I think many of the most popular PC games have code in 32-bit binaries, so Vista 64-bit may not deliver for them. But, as I write above longer term this will surely change.
Anyone remember the move from Mac OS 9 to OS X? sometimes I hear all the complaints and brings back memories.
I don't quite understand the analogy, over the OS 9 to OS X transition most Macs were PPC G3/G4 (both supporting 32-bit), and OS X itself didn't support 64-bit until Tiger (5 years almost after OS X public beta beta).
I think an issue with PC game development is that in order to ensure maximum sales, they tend to be behind the cutting edge hardware and leading edge of the OSs features. That said for the long-term OP may well wish to consider a move to Vista 64-bit at some point for their Macs Bootcamp.