Assemble your rig, check the temps with Coretemp while playing a game or whatever it is you plan to use your rig for, and then decide if its running hot. The stock cooler is usually enough, but it depends on a lot of external factors like your room temperatures.
Which Corsair 650 PSU is it? There's a VS650, TX650, HX650, AX650. Only the last 2 are good.