I think the long term prospects are that Vista will improve the PC gaming scene.
Nowadays the only real reason to buy a good 3d card is to play games. Recently there have been a few other reasons to get a high end card, but a lot of things will crystalize once "Vista-accelerating" cards in new PCs ship in volume.
Just as using PCs now for watching DVD and sub-HD level video is a big use of current PCs, Vista PCs will be used more for HD watching in the future - something that will benefit from a decent video card for smooth playback.
The OS itself will run best with modern 3d accelerated cards.
More and more PCs will have 3d accelerated cards as a result, making a larger potential gaming market. Whether gaming companies actually tap this immediately or not, the long term potential is undeniably there. Sure many of these accelerators only equate to mid or low end 3d performance, but its still a larger 3d market than today.