First of all, Kriegen, please fix the HyperLink.
The differences in frames between XP and Vista are mainly down to two conditions;
A: A typical Vista install has more background process running which will require access to CPU cycles and RAM.
B: Currently, AMD and Nvidia are still getting to grips with optimiseing their drivers to run under the new API.
The final release of Vista came out on the 30/01/2007 and I've been useing it none stop since a couple of weeks before as my local retailer was selling them early. When I first installed Vista I had a 7950GX2 and quite frankly games ran very poorly compared to XP. Prey and Quake 4 (OpenGL) would freeze at autosave points which required ALT tabbing to and from the desktop to fix. The quality of the visuals in games seemed to decrease aswell but worst of all, there was no SLi support at all! My 7950GX2 was crippled as only one of the G71's and half the memory was being used.
However, over the past few months drivers have improved substantially. SLi is now far more optimised in Nvidia's drivers, though it will still only work in OpenGL and DirectX 9 mode. Nvidia claim that there beta 158.43 drivers include DirectX 10 SLi support but when I tried the Lost planet demo I noticed no difference in average frames weather SLi was enabled or not. Hopefully this should be fixed by the time DirectX 10 games are actually released. Aside from the, the only problem I've had is getting a high score on 3DMark01SE as there is no SLi support under DirectX 8 mode. This isn't all that bad as a single modern mid range card should be able to render damn near any DirectX 8 based game anyway.
One thing I have noticed is that alot of people in this thread only have XP listed in their system specs. Either everyone doesn't keep there systems up to date or they're making statements about an operating system they haven't even tried for themselves. I've been through all of Vista teething problems and am glad I weathered the storm. UAC was a god send as I no longer have to disconnect from my netwrok to avoid that bloody WinFixer virus as UAC stops it from installing. Better security, a snappier desktop, practical RAM usage, improved networking features, Terminal Services Client (Business/Ultimate only) and some handy desktop features like SideBar and Snipping Tool have been worht the small 10FPS loss I'm getting today.