This is probably the most overlooked part when it comes to building any system.
I know from experience that cheap PSU's just cause you problems.
I have just bought a coolermsaster from ebuyer. Cost me £71 , but worth everypenny.
This will sound stupid, but trust me on this one, if your PSU feels light, forget it you will get problems under peak load.
If there is a good weight to it, you will probably be ok