Check out this thread: http://www.nvnews.net/vbulletin/showthread.php?t=40217
Firstly, What mobo you Got? Make Sure that u Have a PSU of 350W+.
Download Omega drivers From omegadrivers.net, the latest is 61.77. the latest drivers doesn't always give u the best out of your card
once installed go to the analog display, There will now be a menu for AGP settings which is not available in Standard drivers, Disable the sideband adrressing, Give it a try. Also download latest AGP drivers for your mobo Chipset.
If that fails, try the card on a friends System.
REloading OS could be an option, but that could be unnecessary headache in the case that it fails, so try it on another HD if u have 1 laying around, if Ur using WinXP Pro, format it to NTFS, it's 20 times faster than FAT32.
Hope that one of these solutions could help u out.