PCI-E 3.0 gpu's are backwards compatible with 2.0 motherboards. If you wish to use dual graphics, HD 7670A is the most powerful one compatible and you can't use two. Personally I'd recommend forgetting about dual graphics and get a single higher end GPU that outperforms dual-graphics setup in every way, like R7 260x (which is already twice as powerful as HD 7670A) and up. Make sure your psu can handle the upgrade.
Intel I7 3770K @ 4.5ghz, Cooler Master Hyper 212 Evo, Gigabyte Z77X-UD3H, Gigabyte HD7970 ghz edition, 4x4gb Kingston HyperX Beast 2133mhz, Seasonic Platinum 860, Samsung 840 Pro 256gb, OCZ Vertex 4 128gb, WD Black 1TB, WD Green 3TB+1.5TB, ASUS Xonar Essence ST, ASUS VE278Q, Windows 3.11