My system will boot with one GeForce 7950 GX2 card installed, but not with two GeForce 7950 GX2 cards installed
Running two GeForce 7950 GX2 cards in SLI mode (“Quad SLI”) requires three conditions.
First, your motherboard must be SLI-certified.
Second, check with your motherboard manufacturer that your motherboard is compatible with Quad SLI, or alternatively check the list of known-compatible motherboards. In some cases, you will need to upgrade the BIOS of your motherboard to the latest revision listed on the manufacturer’s website.
Third, you must use the NVIDIA Forceware™ graphics driver version 91.47 or above. Download the latest Forceware driver from http://www.nvidia.com/content/drivers/drivers.asp.