1) Personally I would go with 1600 RAM modules. You can buy faster RAM but you won't really notice a difference.
2) Overclocking is NEVER recommended with the stock cooler that came with your CPU. The stock cooler is designed to keep your CPU within its thermal window under stock frequency.
3) You will gain a slight performance increase if you run your memory in dual channel mode, but once again you may not be able to notice the difference.