Well, it's pretty much clear right now that unless you're playing exclusively DX10 games, Windows XP is still your platform of choice. Much has been made of the current nVidia driver situation in Vista as well, with beta after beta driver release with a final WHQL just hitting last week. The HotHardware guys take the whole mess to the next logical step: what does SLi performance look like between the two OSs?
"Is SLI mode on Vista still two steps behind in performance in comparison to XP? In this article we offer a quick-take assessment of NVIDIA's latest ForceWare driver release, running on Windows XP and Windows Vista installations in SLI mode."