DDR3 1600 mhz or faster, CAS 9 or lower with at least 8 gb's. Usually this is 2 x 4gb sticks. If the price is good, go ahead and get 2 x 8gb sticks(matched pairs, sold as a set). There shouldn't be much issue with different brand names either, all should be reliable.
As a side note, it will also depend on the speed your motherboard chipset can use. Lower end chipsets stop at ddr3 1600mhz...buying faster than that isn't going to harm your system, but you won't get the benefits, it'll just down clock itself to run at 1600mhz.