2933 upwards is going to be fine.
You'll see a fair performance gain just going to dual channel from single without even considering the speed upgrade.
Choosing ram is hard work when it comes to price vs performance, sometimes the speed and latency gain isn't worth the extra financial outlay it incurs.
Also the fact that some ram can be tinkered with settings wise to outperform more expensive higher speed and lower latency sticks (I have patriot viper 2800 here,that once overclocked outperforms my 3200mhz corsair ram) - although on a 320 series board your manual tinkering options are likely fairly limited.
There is so much choice out there, your actual location and ram availability would dictate the choice in my opinion.
Where are you buying from?