Well what are you doing with the computer? if you are rendering large images or videos, you might consider at 16 or 32GB.
if you just plan on gaming any old 2 x 4GB sticks of 1600Mhz / CAS 9-9-9-24 /1.5v RAM should be fine. Corsair, G.skill, and Kingston and Samsung all make good RAM, even the A-data's on sale is a great deal.
edit: good point, the motherboard doesnt support ECC (removed from my post), thanks sniperGod for pointing that out