I know a lot of people are busting to get Vista, and I'm sure it will turn out to be fine, but for now I'd recommend gamers to stick to Windows XP until vista get's it's 1st service pack.
most of the high-performance issues should be sorted by then.
I agree with Chuui on this. Vista security is increased by taking ease-of-use functionality away from the user. They are already releasing vista in 9 or so flavors (media center, portable, home, corporate, etc...) There are 2 MAJOR versions they need... the "I whine about windows being insecure" and "I know what I'm doing, stop locking me out" versions. (XP Home should have never been released)
In answer to the question:...
If you MUST get Vista at launch I'd advise running a dual install so stuff that only works in XP keeps running. At present the biggest issue for gamers will be problems with DirectX 10 and current drivers not working properly with it.
Most games should work at launch, but games like MxO/Quake 4 that push your hardware to it's limits will be the real acid tests.
Just hang back and we'll see if it's another WinNT/ME or if it's a Win98/2k/XP in terms of promise versus delivery. Once people get over the fear of change it should be fine.