I'm new to this so be easy on me ...
You say don't buy a cheap PSU, I was thinking of buying a 800 or 850 watt PSU, but how will a good PSU unit make a difference to the overal PC? I can understand your point for a good CPU cooler, but PSU I don't.
thanks
2 things
firstly, 99% of the power your pc needs is 12v. good PSUs have most of the power as 12v, bad ones push loads though the other rails (5v 3.3v or whatever). so a 850w cheap PSU may have less usable power than a good 400w one.
secondly, a cheep PSU cant provide stable power. you get spikes and dips which is bad news for your overclocking and bad news for the life of your components.
when looking at PSUs, have a look at how many amps they have on the 12v rail(s), that's what matters not the overall wattage
then have a look at reviews of it and see if its quiet, stable etc etc.
edit - oh and when a PSU goes, theres a good chance it will kill other components too.