The spec - http://www.asus.com/Motherboards/M5A97_R20/specifications/ - says
"4 x DIMM, Max. 32GB, DDR3 2133(O.C.)/1866/1600/1333/1066 MHz ECC, Non-ECC, Un-buffered Memory
Dual Channel Memory Architecture
* Refer to www.asus.com for the Memory QVL (Qualified Vendors Lists).
* Due to OS limitation, when installing total memory of 4GB capacity or more, Windows® 32-bit operation system may only recognize less than 3GB. Install a 64-bit Windows® OS when you want to install 4GB or more memory on the motherboard.
* Due to CPU spec., AMD 100 series CPUs support up to DDR3 1066MHz. With ASUS design, this motherboard can support up to DDR3 1333MHz."
So dual channel max 1866, 1.5V and fit under a 212 evo
eg
http://www.newegg.com/Product/Product.aspx?Item=N82E16820231550
You can also check on
http://dlcdnet.asus.com/pub/ASUS/mb/SocketAM3+/M5A97_R2.0/M5A97_R20-Memory-QVL.pdf