Get a 6GB set of 1600. Paying for higher is not worth it, as you will see close to zero difference in benchmarks, and surely no difference in day to day use. 8GB is no good as it would not be triple channel.
If you are concerned you may need more RAM later for serious rendering or something, just get a mobo with 6 RAM slots and you can throw a second 6GB set in later.OCZ gold
is a popular choice, as is the platinum set with 1 lower CAS.