Man RAM is a really tough subject. Take a look at this:
http://www.anandtech.com/show/7364/memory-scaling-on-haswell/7
Now, keep in mind that Intel recommends you do not use a RAM that requires over 1.5V due to Ivy Bridge and Haswell's delicate memory controller. There are many enthusiasts that say it doesn't matter. I personally will not use over 1.5V on my Ivy bridge system.
My conclusion from googling numerous articles = it doesn't make that much of a difference. Get a minimum 1600mhz DDR3, 1866 or 2133 the lower CAS the better, depending on how much you want to spend, but DO NOT go over 1.5V. Thats just my personal opinion. Brand is not as important, as long as you buy a reliable brand such as Corsair, G.skill, Crucial, etc. Don't buy a no-name brand and you should be fine.
Alternatively, wait till later this year when DDR4 boards come out
Edit : In every case, if you are using 1600Mhz or better, for gaming performance your money is best spent on more GPU/CPU power. Get a decent minimum 8GB 1600mhz DDR3, and dump that extra 80-100$ into something that matters. i5 "K" series processor and the best GPU you can find will significantly yield better results than "faster" RAM.