I haven't encountered this before, but I would double-check the BIOS to make sure you have integrated graphics disabled. Also I've read it's important to uninstall drivers from your previous graphic card before installing drivers for a new one.
This would be my sequence of events:
1. Check BIOS - disable integrated graphics
2. Boot up and see if you have the same problem as before
IF THE PROBLEM CONTINUES...
3. Uninstall old 7900GS drivers and Reboot
4. Install new 8800GT drivers and Reboot
Hope this works =P