Tech-Hounds.com take a look at both AMD and NVIDIA's drivers on Vista compared to Windows XP.  Here's a taster:

In closing, its clear that both AMD and NVIDIA still have work to do on their Vista drivers. Based on our experience writing this article, AMD is in a better position than NVIDIA in this regard. First of, the features. Most of the features present in Windows XP are present in their Vista drivers, some not needed anymore are removed and some additional, Vista specific features have been included. There are some improvements we'd like AMD to address and hopefully they are listening. We did expect not all games will run as fast as they did in Windows XP, but we were pleasantly surprised to see most of our game titles (and benchmarks) seem to work just fine, even better in some regard (F.E.A.R). This situation will likely improve with time.

NVIDIA sure have a lot of catching up to do. First one is probably the GeForce 8800 slowdown bug, which they are aware of and in the process of resolving. There's that bug with Quake 4 we encountered, plus the big performance drop with GTR (which also plagues AMD drivers). Clearly, NVIDIA's driver developer team still need to focus on getting performance where they should be. However, that doesn't mean they can't 'reintroduce' driver features already present in their Windows XP drivers. We also like to see some interface touches to make things a little bit easier, those shouldn't be that hard to implement. Now, we know what some people would say. One can point out the fact that we're using graphics card from different generations has a part in this, but that doesn't explain the non-performance aspects such as lack of features on NVIDIA Forceware for Windows Vista.

Is Vista ready for games and benchmarks? We think so, but the question we should be asking is whether or not we (your PC and the drivers) are ready for Vista. We're not there yet, but we're much nearer now than we were at Vista's launch. With Vista SP1 is making the beta rounds, its pretty likely more users still holding out with Windows XP will transition when the Service Pack is finally released.