1. DDR3-1600 is the better RAM to use, even though Sandy Bridge processors (which I'm guessing is what you're using) support 1066 or 1333.
2. Lower latency means better performance
3. The RAM in the link below is optimized to work with Sandy Bridge CPU's using Intel XMP profile.
It also matches the board and is low profile to allow clearance for CPU cooler.http://www.microcenter.com/single_product_results.phtml?product_id=0366817
Just make sure to follow the directions in the manual (page 20) about how to occupy the slots properly for Dual Channel.ftp://188.8.131.52/manual/Z68%20Extreme3%20Gen3.pdf
**If you plan to run 16GB of RAM at some point, may want to just invest in it now while you know you can get the exact same kit. Mixing can cause issues down the line**