Really not worth doing anything at all
worst case scenario, you cpu blows which it wont at those temps, you can get anther one off ebay for nout!
to be honest, I can see about 10 reasons why you have heat probrems with the kit you have got but nothing is worth doing about it.The main reason is that your cpu mostly likely prescot, will run stupid temps and you fan/heatsink isnt that great. But its doing its job. If you want to replace it you can with an aftermarket one, but you really would see much of a difference in your setup. I wouldnt bother as youll be wasting £25+.
A 400 power supply is absolutely fine for your setup, I have a HIPER 580PSU but thats because i have a very good custom built PC well overclocked. You bying that cpu would be like filling your micra up on race fuel on your way to work every day. Ie a complete waste of time.
My advice, leave well alone, ive seen those processors run 90 degree c and never a problem. Most home users cpus run mental temperatures, because of dust buildups in the fan/heatsink and they never even realise.
make sure you have a fan on the front of your case, and rear exhaust fans rely on "through" air flow within the case