Yes, they are backwards compatible. As long as you use all 16 lanes of PCI-E 2.0 you won't suffer any performance loss. Using only 8 lanes in Crossfire with 2 7970s may cause a small performance loss, but nothing too severe.
let me put it in plain words...PCIE gen 3 does absolutely nothing to gaming performance for 7970. 8x Pci 2 is enough to get 99% out of it. If you want to get 1% more..you will end up paying $30-40 extra for no reason.