NVIDIA GeForce 8800 series will not work in PCIe x8 slot
When an NVIDIA GeForce 8800 is plugged into a PCIe x16 slot, it works just fine. However, if it is plugged into an x8 slot or less, the graphics card will not boot. The Problem? Plain and simple incompatibility. Not only are they not listed in the PCI Express Integrators List, but there have been several reports of people sticking their 8800 in a PCI Express x8 slot and being sorely disappointed at the failure to boot. An X-bit labs review also showed that the 8800 series may not even be compatible with some mainboards. Basically, if you are buying an 8800 series, make sure you're not getting a second one before ensuring that you have two PCI Express lanes operating at full speed.