what are you dong with your computer? is it just for gaming or more of a cad/photo shop kind of work?
If its just gaming then 1333Mhz and 8Gb is good enough. If you doing more of the cad/ photo shop I would look at a 2x 8GB 1600Mhz or if you have the cash you can pick these up http://www.newegg.com/Product/Product.aspx?Item=N82E16820226687