Game looks pretty good, and I see only minimal differences between the very high quality (vista) and the hacked very high quality (XP) and since it seems to perform better on XP, at least in the demo version, I think it'd be better to play it on XP.

Damn, the jump from halo 2 is pretty big. But that's to be expected, since H3 is next gen. Looks great IMO, more than enough graphical prowess for my taste. For me, it takes a lot to make a crappy looking game in the next gen. But then again, I'm still content with ps1 graphics.