More RAM or new RAM

I would add 6GB more.

Even if u will get 1600MHz, it will down clock to 1333MHz.

I'd suggest going with a set of 12GB @1600MHz but don't mix it with that set of 6 you have. Then and only then will it clock down the memory to 1333.

I'd also suggest getting 2 sets of 12GB. Mushkin is by far some of the best RAM I have used and actually have their top of the line 12GB set for sale on eBay. Model number 999000.

Mushkin Enhanced Redline with the Ridgeback heatsink.
It did me much justice for the short time that I used it, sadly my mobo only supports 12GB and only 3 sticks.

Just to add also, I'd highly suggest getting new RAM especially from Mushkin, that heatsink you have was not their best.

Go for this set:
http://www.newegg.com/Product/Product.aspx?Item=N82E168...

It runs at 1600MHz and only 1.5v and the timings are really good. I only suggest that only because I don't know if you know about overclocking, voltages, and timings. These timings are nice and tight and at a nice frequency and you won't have any problems of frying your CPU.

The speed difference between 1333 vs 1600 is about 1% in real life.

You don't fry the CPU : )

Its build to work that way, in flex mode if u don't have exact same RAM.