Like yourself, I have found it very confusing trying to understand this subject.
As you say, the different AGP specs apparently run at different voltages (2x = 3.3v) (4x = 1.6v) (8x = 0.8v). And then there's the 'Universal 3.0' AGP standard which is 2x/4x/8x compatible !!!
What I can say for certain, is that I have an 8x graphics card (ATI Radeon 9500) running with a 4x AGP slot. In my case, the mobo is 2x/4x and the 4x capability is enabled from within the BIOS.
Obviously, the graphics performance will ultimately be limited by the 4x AGP throughput - but it really isn't an issue at the moment as 8x AGP is still quite new.
Can't give you any guarantee about the Geforce card - but good luck !!