What speed is your RAM? 1600? 1333? I would be best to get 2 x 4GB, replacing the current RAM, not adding to it. It puts less strain on your board, and is cheaper.
Either of these two: http://www.newegg.com/Product/Productcompare.aspx?Submit=ENE&N=100007611%20600006069&IsNodeId=1&srchInDesc=ripjaws%20x&bop=And&ShowDeactivatedMark=False&CompareItemList=147|20-231-428^20-231-428-TS%2C20-231-426^20-231-426-TS