Quake III Arena (OpenGL), High Quality
Though it's already more than two years old, Quake III Arena is still one of the most popular games and quite important for benchmarking, as it requires both a fast CPU and a decent graphics board for fast gaming action. Here you can see a difference of approximately 2 fps between the fastest and the slowest GeForce3 board. I guess there's no point in talking about "fast" and "slow" in this context!
Again, at 1600x1200, the differences are even smaller.