That has to be at idle. Correct me if I'm wrong, but it's not it possible to get a GTX 580 at 52 degrees C under load with air cooling.
Nothing's impossible, just highly improbable. For all I know the guy could live in Alaska and keep the windows open all night. But yes, that would be pretty low to be believable.
Anyway, to the OP: You might want to think about a new CPU heatsink, but it depends.
If you haven't overclocked it, you shouldn't be getting those temperatures even with the stock cooler. So chances are your problem is that the heatsink isn't sitting right, or you used too much thermal paste, or the airflow inside the case is obstructed. Those are all easy to fix for free.
If you've overclocked it, the stock fan is generally not going to do a good enough job and you'll start to see temperatures like this. So then you'd want a better cooler. Although at idle, I still don't think you should be getting those temperatures. So check for the above problems first.