There are a lotta cards that support PCIE 3.0 .like the gtx 700 series , r9 series. ..
The problem that no mobo manufacturer includes PCIE 3.0 on an AM3+ Is that PCIE 3.0 isn't supported by any AM3+ chipset... Even AM3+ most powerful 990FX chipset. doesn't support PCIE 3.0...
PCIE has both forward ans backward compatibility. .. you can install a pcie 3.0 card on a pcie 2.0 and vice versa... and the performance is almost same .just a couple of fps drops..
Asus recently released the first Am3+ mobo with pcie 3.0...
http://www.asus.com/Motherboards/SABERTOOTH_990FXGEN3_R20/