I had an ATI x1800xl that didnt work with pci-e 4x, however it works fine with my x850xtpe, its a bit of a minefield as to what works.
I read that x1800GTO's would work though, oddly enough. As will the x1600 series. With regard to Nvidia, i have no idea which ones will.
Theres nowhere documenting exactly what cards will work so its best to steer away from the 4x motherboards.
I think i have encountered the full speed of pci-e 4x as playing battlefield 2142 on max, which runs fine, when theres alot of stuff on the screen it doesnt lag it kinda hangs sometimes its really weird.