I have the latest drivers for both the cards and the mobo. The NVIDIA Control Panel detects both cards, as do Device Manager and GPU-Z. GPU-Z shows that the two cards are _slightly_ different - one is a Revision A2, the other a Revision A3. Shows NVIDIA SLI as Disabled. Do not see SLI options in Nvidia Control Panel. What. The. Hell?
Mainboard: MSI P55-GD65 LGA 1156
GPU: Nvidia 8800 GTS 640 MB
OS: Windows 7 Professional x64
Quick Google search turns up nada. I can't find any hacked drivers or any information on how to flash the card. I feel like I must be missing something here, but apparently this is a common problem. The Nvidia forums are littered with posts like this. Grrrr....
Also of note: GPU-Z shows one of the slots only running at PCI-E x1. I thought this mobo had two x16 slots...
Definitely something wrong there. They should both say PCI-E 2.0 x16 @ x16 2.0.
Have you tried installing only one card at a time and running GPU-Z to confirm that they are both capable of running at x16 2.0? If they both run at x16 2.0 individually then I would suspect a problem with the second x16 slot.