I'd stay away from a 64 bit O/S. There are incompatability issues with 32 bit applications, application installers and 32 bit hardware.
As you go to SLI and Firefox configurations, it almost becomes necessary to go with a 64bit OS or limit yourself to 2 gigs of memory.
My experience with 64 bit vista says that most programs will not have a problem with a 64 bit OS. I have yet to have one fail on install. The only game I've had a problem with was ArmA, which had a problem with any system that had more than 3 gig of memory (not necessarily 64bit specific). When it comes to drivers, however, that's a different story. Very few of the wireless and networking companies had stuff on the market to support it. But, I imagine that will get better over time. I still can remember how many 16bit network cards there were when Microsoft introduced XP and the uproar that ensued.
Also, at work, our custom made applications (written by programmers here) are less likely to be compatable. I seriously doubt any company that has any customized software will venture into the vista world anytime soon.
Overall, though, if you want to go with 8gig of memory and SLI or firefox configurations, you don't have a choice.