For discrete GPU users, recommending any kit over another is a tough call. In light of daily workloads, a good DDR3-1866 C9 MHz kit will hit the curve on the right spot to remain cost effective. Users with a few extra dollars in their back pocket might look towards 2133 C9/2400 C10, which moves a little up the curve and has the potential should a game come out that is heavily memory dependent. Ultimately the same advice also applies to multi-GPU users as well as IGP: avoid 1600 MHz and below.
but i suggest reading the whole article:Memory Scaling on Haswell CPU, IGP and dGPU: DDR3-1333 to DDR3-3000 Tested with G.Skill
notice that is all haswell platforms, when it comes to sandy/ivy the scaling steps down a notch and 1600 ram is fine but 1866 does have some increase with 2133 or above not being cost effective.
Sandy Bridge Memory Scaling: Choosing the Best DDR3Edited by looniam - 11/2/13 at 2:49am
I think we confirmed what we pretty much knew all along: Sandy Bridge's improved memory controller has all but eliminated the need for extreme memory bandwidth, at least for this architecture. It's only when you get down to DDR3-1333 that you see a minor performance penalty. The sweet spot appears to be at DDR3-1600, where you will see a minor performance increase over DDR3-1333 with only a slight increase in cost. The performance increase gained by going up to DDR3-1866 or DDR3-2133 isn't nearly as pronounced.