Need some help and info on voltage of some 4x/8x AGP cards!

I have a motherboard which supports only 1.5V 4x/8x AGP cards. The mobo is KT4V MS-6712 v1.0 (G52-M6712X1). I've tried using a Geforce 7800 GS with it and it refused to work on this mobo. The computer store guy, who works with computer hardware every day, wasn't able to get it to work on my system either. In the end he wasn't really sure what the problem was since he had only one or two hours available to spend with my computer. He also tried the card on some other mobo and it worked just fine. Fortunately, I was able to return the card to him and got my money back. I suspect the reason for Geforce 7800 GS not functioning properly could be different 4x/8x AGP voltage on that card. Another possible reason why it didn't work is that according to dxdiag.exe and Catalyst drivers the currently installed graphics card (Radeon 9700) isn't running at 8x AGP in Windows XP. Changing settings in BIOS doesn't help to get the 4x or 8x AGP option running, even though both the mobo and the Radeon 9700 support 8x AGP. Actually, Catalyst claims that AGP is completely Off, not even at 1x, and dxdiag.exe claims that "AGP Texture Acceleration" is "not available, while in practice even games like Doom 3 run at maximum or almost maximum graphics very smoothly. I was hoping I could upgrade the whole computer, but now it seems the best option would be to upgrade just the graphics card to some card from ATI's X800/X850 series, and upgrade the whole computer after a year or so.

Here are few questions:
1. How to get 8x AGP running on that mobo?
2. Would an X800/X850 Pro or XT card work on this mobo or will there be the same problem that was with the Geforce 7800 GS?
3. What voltage is on Geforce 7800 GS and X800/X850 AGP cards?