What do you mean by it not being detected?
Does it show up in the BIOS? Does it show up in windows? does you monitor show anything when plugged into the 7950?
If you installed with integrated graphics, sometimes it stays there even after switching to a video card. Try plugging them both in on your monitor if it have multiports. or just try to disable the onboard or tell it use video card (usually PEG).
Luck
-=Mark=-