Have you tried leaving the monitor hooked up to the on-board VGA connector while the new AGP card is installed?
If you get picture from the on-board still then that means the AGP is not configured to be the primary card. This setting can usually be changed in the BIOS.
Also, have you tried to install the video card in another computer at all?
Find out if its the computer or the card itself to narrow down the problem. I've had alot of trouble with Xtasy video cards in the past. I've had alot of bad video cards right from shipping.
And last thing I can think of is exactly what you said earlier. It can possibly be incompatible with your PC. This is pretty rare, but it does happen from time to time. Even if your computer meets all requirements for the card.
