If you can spare the extra for the 620w, i think that would be a better option - just as quiet (minor variance), but alot more capable on the 12v rails where you need the power.
If i O/C it will only be to about 2.8-3.0GHZ i probably wont until i can afford a new PSU.. i new it would be tight... but it should handle it right? 'cause i just ordered the Q6600 lol.
It'll probably be fine for now, but as Mr. Smith said, would not give you much headroom. PCs are using more and more power with every new generation of technology, especially videocards. For now, I would go with at least a 650W unit.