It really depends on what you're trying to do... and the video card itself is more of a limiting factor than the CPU.
Some sample things I've done ran better on a P400 with a 16MB voodoo3 card, than on a 2GHz system with craptacular onboard video adapter.
Some sample things I've done ran better on a P400 with a 16MB voodoo3 card, than on a 2GHz system with craptacular onboard video adapter.