i have a corei7 oc'd to 3.6ghz with ONE front case fan atm and i haven't had a single problem with heat. Im using a corsair 750 psu.
cpu idles aroudn 40c and load around 70, another front fan would prolly drop that a few degrees. as far as the psu goes, these temps are about avg for most people from what i've seen so i can't see that my psu is causing any extra heat concerns.
using the antec 300 btw