what about the other way around? a 2x/4x card on an 8x board? I have a Radeon 9000 Pro, which I believe is 4x. I plan to put it into a new system, with 8x agb. possible?
I'm sure that the 2x card can't be used on the 8x board (NEVER do this) because the voltage of 2x card is higher than 8x card. If you try to do, your display card can ''boom" . But I'm not sure about the 4x card.
Most motherboards nowadays (as well as many first-gen AGP 8x and 1.5V-only AGP 4x motherboards) have had the "idiot-proof" key. Usually when there was no key, the motherboard would simply detect an AGP 3.3V card as a video error.
I suppose though, that one could remove the bracket of an AGP3.3V card and put it into the 1.5V slot backwards, but someone who'd want to do that would truly have a questionable mind.