RAM and GPU question

[DrPop]

I'm *slowly* getting things ready to build up the two 3930K mobo/cpu combos I got from Bryan. I just bought the two SSDs today. I still need to get RAM and PSUs and one more GPU.

RAM question: will I lose any speed if I just buy 2 sticks of 8GB DDR3 and call it good, or is there any crunching advantage to using all 4 channels and doing the 4x4GB setup that I'm running in Lynx and Cheetah now?

GPU question: Any good mid-range GPUs come out recently? Maybe something around the GTX 980 performance wise and of course more GFLOPs per Watt is always helpful.
EDIT: Only AMD GPU that looks tempting is the R9 Nano. 8,192 GFLOPs / 175 Watts. Not too bad... ~ $350 on ebay.
Nvidia side: GTX 1070 looks like the price/performance sweet spot. 5,780 GFLOPs / 150 Watts. ~ $380-ish.

Thanks for any suggestions!