(ie more than 4mb graphics used on a 4mb vidcard), expect to see some major slowdown. And of course that depends also on the PCI/AGP bus speed... 8xAGP is a wonderful thing. =]
Except that bottom of the line 8x AGP cards have at least 64, and usually 128, megs of vid ram. If you are swapping that every frame, shame on you! :)