RAM

My girlfriend just bought me some more RAM for my computer (4GB DDR3 1600 2 X 2GB) but I previously had 4GB DDR3 1333 (2 X 2GB) already. Basically, I want to know which setup is best for gaming. I previously looked up this question and found many similar questions, but most were about completely changing out their RAM, whereas I want to know if using ALL of my sticks (8GB @ 1333) or just plain 4GB of 1600 would be best. I am using an EVGA 790i FTW motherboard, just to let you know.