Vista RTM & Gaming -- thoughts
I installed Vista RTM on my Mac Pro (yes, legally). I did some benchmarking using World of Warcraft against XP, Vista RTM and OSX. Here are my results (YMMV):
I have the ATI X1900XT card for the Mac Pro, and 2gb of RAM (this is the 2.66mhz Mac Pro)
XP (Using latest Catalyst drivers): While sitting in Razor Hill, I got 64fps (there is a known bug in WoW on XP with Intel Dual Core chips locking at 64fps)
Vista RTM (Using Beta drivers): While sitting in Razor Hill, I got 54fps
OSX: While sitting in Razor Hill, I got 101fps.
So, the point here really is that with Vista, you will take an FPS hit (because of the bug with WoW and Intel Dual Core chips it's hard to say how much, but at least 10 fps).
I do think that gaming in Vista will get better once graphics card drivers improve, but it will be a bit yet.