1. The higher the speed and lower the latency the better. Lower latencies are more important than faster speeds, although frankly, neither are noticeable, only in benchmarks. You will want a triple channel 6GB DDR3 kit, and if possible I suggest 1600 MHz and CL7 RAM.
2. I'd personally go with Kingston HyperX. Although just buy the one that is at least 1600 MHz and has lower latencies.