Games that are multi-core aware will benefit from the improved scheduler in Snow Leopard, for both game and non-game threads. Games that are NOT multi-core aware will not benefit as much, but might see improvements purely from better scheduling of other tasks running at the same time as the game. The video card performance, if it improves at all, will be due to driver improvements, and not due to technologies in SL. As far as drivers for video cards not available before goes - that's entirely dependent on what Apple decides to do, since, for some strange reason they choose to write the drivers themselves rather than letting AMD/ATI/nVidia do it for them.
This is just an educated guess, I may be completely wrong on some of these points.