Hmm.. Okay, let's see.. you could remove the side cover of your case as you already did, and point a desk fan blowing air inside, it's not the greatest solution. but it'll keep the temps down for a while, have you changed the thermal compound on the video card? have you checked if the cable management is blocking some of the airflow?
Anyway, the desk fan pointing at the case at max speeds will keep the temps down, at least temporarily. and it won't hurt your computer
EDIT: the 58ºC is on full load or idle?? if that's an idle temp, we have a big trouble here, 58ºC idle on a processor is a bit too high.