I know you're worried about a 3.0 card running slower in a 2.0 slot. Most games today still don't need more bandwidth than the 2.0 slot provides.
So when you compare performance of a 3.0 card in both slots, this is the resulthttp://www.overclock.net/t/1188376/hardwarecanucks-hd-7970-pci-e-3-0-vs-pci-e-2-0-comparison
And you can't compare graphics cards by revision of PCIe they can use. If a card uses PCIe 3.0 it would be newer and often more powerful than a card that uses PCIe 2.0 anyway. You seem to have a misconception about graphics card power and compatibility. Both are separate issues and I suggest you google/youtube to learn more about them.