Neither the timings nor the frequency of DDR3 RAM makes a whole lot of difference in system performance. You do NOT want to buy 8 GB. to add to your system. RAM is so cheap right now you should buy a Matched, Tested 12 GB. Kit as mixing of RAM is never recommended and unlikely to work without issues.
You don't want to mix ram just like beenthere said if you can help it.
If you want more ram than 6 gb than get two of these and sell your current ram for like 20-30(maybe more) bucks then you can get 16 gb of ram. IMO unless your doing photo editing/3D editing things than I wouldn't recommend more than one 8 gb set. $39.99 for 8 gb of DDR3 1333 RAM is good value.
Here's a good deal but it ends today so jump on it. It's 2x4gb set you can get two of them for $79.98(with $10 worth of promotional codes) with free shipping
Ooh and by the way, my mobo is an ASUS M4A89GTD Pro/USB3 which work only in dual-channel operation. Am I doing something wrong here by buying three sticks of memory or should I get four stick ? Or maybe keep 2 of my 2gb stick then add 2 x 4gb stick in the remaining slot ?