1. Lower Cas Latencies are always better. I haven't seen any direct comparison between CL3 and CL2 but its expected to have a considerable boost in FPS, according to Tom's guides.
2. Having two sticks of RAM is kinda better because you can make use of Dual Channel mode, which only works in pairs. But then again, you would need two sticks of the same brand and speed, not to mention a processor/motherboard which supports dual channel, in order to make full use of its advantages.
PS: Dual Channel doubles the bandwidth of your memory, which is a good thing.