550 would run it no big deal.
Rugger recommended the XFX 550, and I concur since it's a great PSU and you can rest assure that it will power the machine. If you are one for upgrades (we all are at heart) go for 650, you could probably do xfire or SLI if you plan it right, def. not with high end cards though.
i just got screwed; that psu is not on the pcbuilder part of the site , can you name me an another psu from the ncix.ca pc builder part? :s.