linkage
If you notice AGP 8x is a theoretical bandwidth of 2100MBs, while PCIE 8x is 2000MBs.
So even at 4x AGP you are still looking at the equivalent of a PCIE x4 slot. Which
here, shows it works fine. You synthetics will suffer, but real world will not.
Honestly, you can check
here, and notice Intel states that two devices can be connected to the AGP 3.0 bus. Though it leaves the timing issues to the system designer. IE AGP 3.0 is capable of running a dual agp (dual 4xPCIE) equivalent setup, with the current specs.
Edit: When I moved from a 2500+ xp to a 3000+ A64 I noticed on a 9700pro a big performance jump. Everything got nice and smooth. When I added the 6800gt I could ramp up the IQ but the game ran fairly about the same as before. So on a 6800gt upgrade, YMMV with an older processor as stated prior.
Check out
this link to see where you current processor would compare itself to against a A64. An inexpensive 754 or 939 board, with a 2800+ or 3000+ may get you better results than a 6800gt with your current processor. As the cost of that upgrade will be similiar to the 6800gt bump.