Get another of that same Corsair kit for a total of 8GB. You'll be able to set the RAM to 1600 in the BIOS.
The one bad thing about Corsair is that they tend to spec their RAM at 1.65v and that's at the very high end of Sandy Bridge compatibility. You may still be able to use the 1.65v modules, but it's not guaranteed. For true compatibility you need 1.5v spec RAM.
Using a 2x4Gb set will be the most stable for overclocking and RAM prices are so low right now that you might consider upgrading to a completely new kit. Maybe something like this?: G.SKILL Ripjaws. They are 1600Mhz CAS9 and run at 1.5v, which seems to play nicer with Sandy Bridge. Plus G.SKILL is probably the best name in memory (in my opinion G.SKILL, Mushkin, Corsair are the top three)
If you do want to re-use the Corsairs that you already have, then your choice is simple. You should get an identical 2x2gb kit. Mixing and matching RAM is never recommended.
Also, you're motherboard will support memory speeds up to 2133 Mhz (Source)
Also Also, if you are using a big cooler make sure it has enough clearance for whatever ram you select. It looks like you are using some high end parts there and a lot of the top-end air coolers hang over the RAM slots.