It clocks down to save power when not in use. See the little guage that says "Activity 0%"?
When you start playing a game, it will go up to it's full speed.
Why you have lag, I have no idea. Do you have the latest drivers? Did you properly clean out your Nvidia drivers before installing ATI drivers?
Gpu could be overheating as 54c is my max temp on my HD5850, yours shows 54c at idle. (possibly you just shut a game down?) Alt-tab out of a game while playing and see what the temps shows then.
Another thing is Rosewill PSU's are complete garbage, you might as well have built your own psu. lol