PCIe 3.0 card will work in PCIe 2.0 boards with little performance compromise, to be honest you probably wouldn't even be able to notice the difference. Now that that is out of the way, when you say "single slot" do you mean the entire card cooler and all have to be "single slot" or do you mean just one card, no SLI/Xfire?
Assuming you mean the latter, From AMD = HD7970 From Nvidia = 680 <--(690 has 2 680 cores on one PCB so technically it fits your criteria as well).