Would it be possible to overclock two DDR3-1333 sticks so I could use them together with two DDR3-1600 sticks?
(This is, of course, because I have two unused sticks already.)
What speed do your RAM DIMM slots run at?
If they are limited to DDR3-1333, when you put in faster RAM it will automatically downclock.
If it is DDR3-1600 or above, then hope that you have a motherboard that will support RAM overclocking. It's not uncommon though.
Just go in and set the speed, but make sure it's stable.