Wow, I just tried it in Vista and there is a HUGE increase in performance when running at maxed settings! (all sliders maxed, with highest AF set and 16x forced in the control panel... no AA)
I didn't expect much but it really is noticeable!
In the area that was pretty slow (on top of the large building that is under construction) I get around 28fps in XP when overlooking a certain area. In Vista I get 40fps in the same area under the same weather conditions!
I've been seeing a 5 to 15fps increase depending on the area and the game feels MUCH more playable under Vista.
Anyone know why this is? Maybe it has something to do with DirectX 10 even though the game itself wasn't programmed for it. Or maybe it has something to do with better multi-threading support.
I'm extremely impressed. I'll definitely be using Vista more often now since STALKER is going to be my #1 game for a while.
EDIT: BTW, in that area on top of the building I tried 1600x1200 and 2048x1536 and had the same framerates. I prefer 1600x1200 though since the grassy areas tend to get a bit mouse-laggy (especially while zoomed in with a scope) at anything higher than that.