KV8 Pro and KV8 Max3 Video Card Problems

Just bought and XFX 4650 AGP card and a HIS 4670 AGP card to put onto these motherboards. I know the motherboards are quite old (circa 2004), but they both have the 8x AGP slots and the current Bios have been updated about 2007 for both. But I am not getting any picture off of these cards. I also have a Visiontek 2600xt that I got off Ebay that doesn't work in them either. I thought the 2600xt card was bad, but Im beginning to think not. It does the same thing, no picture. In one of the computers is a 550w power supply (its a generic one), but I think its good enough to power at least a video card. I know the motherboards are both good because I have a ATI 9800xt video card that works perfectly in them. So do you think the motherboards are just simply not compatible with new cards? Can't find anything on the internet that kinda relates to this problem. Any replies are appreciated.