50 - 58 is fine.
You should of seen mine after a local shop put my pc together. Aparently it wouldnt need a new layer of thermal compound on it (dumb azz's). It was running at 70 to 78, eeek.
So, i bought a new fan (THERMALTAKE VOLCANO 7+ VARI SPEED) and some thermal compound (ARCTIC SILVER III COMPOUND). After reading the instructions of both web sites and deciding to have a go I went for it. Mission succesful.
I can now choose the operating temperature, and 50 - 58 is fine. I can have it at 42 - 45 but its a bit loud and i like to hear myself think.