It's quad channel so for the best performance you need to install 4 or 8 sticks of RAM.
How much do you want to spend?
I really have no idea if faster RAM makes any difference with Sandy Bridge-E.
1600MHz kits:
Kingston HyperX Genesis 16GB (4x4GB) DDR3 PC3-12800C9 1600MHz Dual/Quad Channel Kit
G.Skill RipJawsZ 16GB (4x4GB) DDR3 PC3-12800C9 1600MHz Dual/Quad Channel Kit
1866MHz:
G.Skill RipJawsZ 16GB (4x4GB) DDR3 PC3-14900C9 1866MHz Dual/Quad Channel Kit