First: I would definitely suggest a different RAM kit -- specifically, one rated for 1.5v spec. If your CPU fails and Intel determines the cause to be running RAM at 1.65v (which is out of specification for all Intel CPUs since Nehalem) then the warranty is null and void. You can safely run RAM at 1.35v-1.6v (and still be within warranty) on Sandy Bridge CPUs.
Capacity matters the most, up to a point. Once that point is reached, further capacity makes almost no difference.
Frequency matters more than latency.
For Sandy Bridge and the gamer/overclocker type of user, 8GB of DDR3-1600 CL9 RAM is the sweet spot. If you want a few more points on synthetic benchmarks, go for higher frequency and lower latency ... but you're really wasting money going "better" than 1600 CL9.