|"I know that 1600 is blazing fast"|
Tests show that memory speed & timings makes very little difference on an AMD system except under certain circumstances. The difference between DDR2-800 & DDR3-1333 is minimal. I believe DDR3-1600 would default to DDR3-1333 anyway because that is all the memory controller in AM3 CPUs officially support unless overclocked. But since you apparently have a Black Edition CPU, overclocking should be done via the multiplier anyway & that has no effect on memory speed.
Here's a couple of clips from an article at Tom's Hardware called "DDR3 Memory Scaling On AMD's Phenom II X4" :
"Our results show that faster memory and quicker timings both introduce performance advantages that are mostly visible in synthetic benchmarks, while the impact on real-life performance usually is small. Games benefit just a little, and you need applications that intensively tax the memory to find serious performance benefits."
"We couldn’t resist doing one more benchmark run at a slightly overclocked processor clock speed, which serves as a great example in the event that you decide to spend extra money on a faster processor rather than on premium memory. Overclocking the Phenom II X4 955 from 3.2 to 3.4 GHz results in a hypothetical Phenom II X4 965 at 3.4 GHz, which we might see some day. We used the slow DDR3-800 memory setup to show that a processor speed bump can actually do much more than any upgrade in memory speed."
Your board has 4 memory slots & you already have 2 x 2GB DDR3-1333 installed, it would be foolish to replace it with 2 X 4GB. Just add 2 x 2GB DDR3-1333 to what you already have & call it a day, or add the 2 x 4GB for a total of 12GB.