I am thinking about upgrade from 6 GB DDR3 to 12 GB DDR3 and move from 1.33 GHz to 1.60 GHz

Right now My PC had 6 GB DDR at 1.33 GHz (triple Channel Memeroy). I am think about move up to 12 GB DDR3 at 1.60 GHz (4 GB DDR3 x 3) . Should i buy another 6 GB at 1.33 GHz to add to my current PC? OR buy 12 GB DDR 3 at 1.60 GHz to my PC ?  Keep in mind, My motherboard, ASUS P6X58D-E has XMP support for high speed memeroy DDR3 at 1.60 GHz. I need your advice on this issues. Thanks.

what OC clock ratios are you running? 

Hey, I am not sure what you means by that. My MB BIOS setting "auto" default at 1.33 GHz. The ram set speed at 1.33 GHz that I did bought from about 8 months ago when i built it.  My i7 990X running at 3.46 GHz and sometime I overcloced it at 4.40 GHz and it did great job with running stock speed on memeroy at 1.33 GHz.  My RAM is Kingston ValueSetect 6 GB PC10600 DDR3 at 1.33 GHz and has Cas Latency is 9 and volts at 1.5 v. I did check in my MB book that able to support up to 1.65 volts when use DDR3 at 1.60 GHz.  If I go with my idea on 12 GB DDR3 at 1.60 GHz  and am willing go ahead to buy water cooling system too to keep cooling when overclocked over 4 GHz. Thanks for your relay. I will check on the post later.

While having "quicker" is nice I doubt you would notice. If you plan on running 4.0ghz then it might be worth it. 

Hey, Thanks for relay back to me. and thank you for your help. I think it will be best performance on my PC. and I'm move on with other stuff and have a great day