How about PCIe 1.0 or 1.1? Should be compatible but slower, right?
Also 3.0 is not required even though they support it. If your about to buy a motherboard I would get 3.0 (every mobo has it now anyways).
However, having 3.0 will not give you a significant increase in speed or anything at this time. It is not yet utilized by games etc because our graphics/cpu are powerful enough for any game. As game tech improves so will the improvements of pci 3.0 which are marginal at best currently.