You'll have it known that the 8x AGP still have so much excess bandwidth that even a 4x accelerated port is ample for anything from 6800 and below.. Even at its peak pump rate, None of todays AGP will actually saturate the 8x bandwidth.. no even close TBH..
So I doubt you'll see much hit when going down to 4x on a humble 6800GS..
Ohh, also its a 6800Ultra..
, and thats if you're lucky enough to get an early NV40 core that unlocks (though I have been reading from ppl that did actually unlock)