The default RAM frequency on AMD Phenon II CPUs is 1333 Mhz. The mobo that you have supports AM3+ CPUs meaning Bulldozer/Zambezi and similar in addition to AM3 CPUs.
The Zambezi CPUs have a default RAM speed of 1866 Mhz. so if you ever plan to use one of these CPUs you should buy 1866 Mhz. RAM for best performance. The 1866 Mhz. RAM will still work fine at 1333 Mhz. or 1600 Mhz. if you're running a Phenom II CPU.
I agree with everything that BeenThere has said but I have heard that differences between timings and frequency have a very very very low impact on the real-world FPS that you will be getting (maybe a couple of frames per second). Get a brand that you like and what's in your budget. Personally I tend to lean towards Corsair because of their lifetime warranty and their exceptional products. This is a nice article for you: