What you need to do is enter BIOS, by hitting repeatedly DEL, until you're in the menu. Then, look around for the primary display adapter. change to AGP and then, look for video memory sharing size. Set it to None. It should make the AGP card detected. Are you seeing something from the card?
i have it set to AGP i will try to change the memory sharing size to none and i will get back to you.
Now I went ahead and installed my 7800gs and then went to BIOS and set my share memory size to disable. I then had it save the setting and it rebooted, but there was a problem when it rebooted i had nothing but a black screen with a bunch of symbols with color and it wouldnt fully boot to the desktop. I then took the 7800gs out and put in the ATI and went to BIOS and put setting back to default and then rebooted and it went to desktop fine. One other thing is when i had the 7800gs in and had my monitor hooked up to the back of the 7800gs the monitor wouldnt even come on it stayed in sleep mode it looked like but when i hooked it to the back of the mobo it was fine. Between the share memory size and the graphic win size which one is the right one that i need to be messing with?
yeah I was about to say the same thing, sounds like the 7800GS isn't working at all. Thouhg I'd try it on a different computer before returning it, just to make sure.
And you have plugged the power cable into it?