agp and pci-express are the same. Just ones an intel invention (agp) and pci-e I cant remember who invented it. They work the same, pci-e has more potential but at the current time they work the same. And with agp 16x around the corner it is just going to mean both stay level with each other.
I think you've got the voltage thing backwards. PCIe delivers more voltage than AGP. My AGP 6600GT requires an extra power input, whereas the PCIe version of the 6600GT does not.