I think the best option for you would be to buy a whole new dual channel kit of (8gb x 2) i would say that corsair is again the way to go so. This kit is great plus it is low profile so you can fit large coolers over your cpu.
You should buy a 4x4GB kit, the x58 chipset supports tri-channel memory (which is why three of the ram slots have the same color). If it runs how I think it will, you'll have three dimms in a tri channel config with one dimm "floating". If this doesn't work then you will have two sets in dual channel, which will give you a general speed increase over just two dimms.
It's also worth noting that some motherboards (especially lower end ones like yours) may only support the max memory if all slots are filled. If this is the case, your max individual dimm size may be 4GB, not 8GB.
I would recommend the Corsair 4 x 4GB LP Kit. Note that you may be able to find it cheaper if you search around. Most other brands would be acceptable as well, I would look at a cheaer 1600MHz 4x4 kit if you can find one.