Darn Compaq... dual monitor issues

I know... it's a Compaq...
It's a Presario 5280 with ATI 3D Rage LT pro built onto the motherboard. The only PCI video cards I have are a Ttident ProVidia 9685 and a SIS 6326 (it says AGP on the card, but it's a PCI card??). Oh, and this darn machine only has two PCI slots. Whenever I put in one of the video cards, the video switches to that card/monitor combo and disables the onboard card. Under device manager, it says it can't start the device. I've even went so far as installing both card at the same time, and it reverts to the Trident on the outer-most PCI slot (disabling the other two). Tried updating the drivers, etc. Nothing works. OS is Windows 2000 Pro. Any ideas?