Abit KT7A problems with ATI 9800 pro and Geforce FX 5900

I'm having a hell of time with a system of mine. Its a home built Abit Kt7A raid running rock solid 1.3 Ghz Athalon, 1.5 Gig ram and a 64 meg Asus V8200 ( GeForce 3 ) with a 300 watt power supply. For these tests I'm running at 100 fsb and 33 pci.

Recently I've been playing around with some video programming which requires a huge amount of texture and I'd like to replace the 64 meg video card with either a 128 meg ATI 9800 Pro or a 128 meg Nvidia FX 5900 both of which I have on hand.

According to AGP Compatibity the Abit KT7A is a "Universal AGP Motherboard" and should support AGP 1x - 4X at 1.5 volts. Both the video cards are double sloted and look like they should work.

The problem:

The ATI 9800 Pro is not recogized by the system and gives me the beep beep beep until I pull the power. I've tried setting the AGP-4x Mode to Enabled in the bios hoping to force it into 4x mode. No luck jus tbeeps.

The Nvidia FX 5900 is recognized by the system but causes system reboots as if cycled the power. Usally getting a WinXP error saying their was a critical failure. Once or twice I've seen an NVidia dll error right before a forced reboot. When I swap my Asus V8200 GeForce 3 back in every thing is stable again. Bios was set to default values.

I've checked the video cards in another machine thats running with a lowly 250 watt power supply and both are stable in that machine. I'm tempted to swap power supplies but I don't really think its a supply issue.

Any one have any other ideas to try?

Thanks!