Both cards are PCIe, 6600 (original) and 6700 (new) Radeon, different manufacturers, on a Windows 7 PC.
I tried moving the monitor cable between the two and still couldn't see anything. So, for now, the newer (and faster) card is back out and standing by.
Any suggestions?
Try swapping the cards PCIe slots. Move the monitor between the two until you find a screen that works. You'll probably have to uninstall and reinstall both drivers. Also, for some reason, you may lose your Internet connection through the process. But you should be able to get everything working.
+1 for lulz