Re: Which vista to buy...
I run x64 vista and have never run into a game that I couldn't play. As for unsigned drivers, yes this can be an issue, especially for RivaTuner and other overclocking software. Yet, there are 2 that I know of that don't require any reboots or driver signing, and they are EVGA's and Gainward's, both available at their respective sites.
The one app I do remember not having any support for x64 was the Microsoft Fingerprint Reader that I used in x32 before upgrading. They say they will never add x64 support for it's drivers, so aside from that, x64 support is A-Okay.