Еве и подетално
"As we mentioned before, installing a PCI Express 1.x card in a PCI Express 2.0 compliant slot will result in PCI Express 1.x speeds. The same goes for installing a PCI Express 2.0 card in a PCI Express 1.x compliant slot. In every case, the system will operate at the lowest common speed with the understanding that all PCI-E 2.0 devices must be engineered with the ability to run at legacy PCI-E 1.x speeds.
Unfortunately, there have been some reports of new PCI-E 2.0 graphics cards refusing to POST (Power On Self-Test) in motherboards containing chipsets without PCI Express 2.0 support. We can assure you this is not by design as the PCI-SIG PCI Express 2.0 Specification is very clear on the issue - PCI Express 2.0 is backward-compatible with PCI Express 1.x in every case. With this in mind, and knowing that some PCI Express 1.x motherboards have no problems running the new graphics cards while others do, we have no choice but to blame the board in the case of no video. Unfortunately, this makes it difficult to determine if problems await during the upgrade process; you must first consult with others that use the same video card/motherboard combination. Thankfully the cases of incompatibility seem to be few and far between."