In theory a good 300-350W powersupply will do.
You wont get better performance with a 'stronger' powersupply, you will only be sure that it can give your system enough power and that you can put other components in it without having to worry of to little power.
My dual-PSU PC is so powerfull that the neighbourhood dimms when I turn it on
Some 500w psu's only put out 350w. Some put out 420w.
On the other hand, a whole bunch of 300w psu's only put out 250w. Some put out 310w.
This is the game you play. At the very least, no matter which one you choose, make sure that it's <i>name brand</i> is listed on the processor manufacturer's website. This is like, the most bare minimum QA that you could ever possibly hope to expect. It's not really giving you "peace of mind", per say; it's just to weed out the crap that a 5 year old would say, "Wow! That is some [-peep-] hammered dog [-peep-], right there!"
"I personally think filesystems should be rewritten from scratch every 5 years..." --- Hans Reiser
Ditto to the last post. Try Fortron/Sparkle, Antec, Enermax, TTGI(Directron)and of course a few others. The Fortron/Sparkle units are probably the best bargain. Read Tom's review of 21 PSU's and check out the Conrad and Verax. Both model numbers start with "FSP"= Fortron Source Power. You'll be glad you did.