3.0 cards in 2.0+ slot will work at 2.0+. 2.0 cards in older 1.0+ would work at 1.0+ . Simply, all of them are backward compatible. You could run it on PCI-e 1.0 x8; it would be bottlenecked but it would work. But at your case, I guess any single 3.0 graphics card could work normally on a Mobo with with PCIe 2.0 smoothly.