Plug your components into here:http://www.antec.outervision.com/
Don't forget to include things like capacitor aging which should be low for a hi end PSU and higher for a low end.
While it is true that PSU's are the most efficient at 50% load, that does in no way mean that you "NEED" to operate at the BEP (best efficiency point). Your 650 watt PSU, may be say 90% efficient at 325 and 88% efficient at 500. That being said, given current price structures, I would never buy a PSU less than 650 watts.
I'd look for an Antec EA-650 / Antec TP-650 or Corsair TX 650 .... 500 watt PSU's of comparable quality are usually only around $10 cheaper.....most are not of comparable quality even if they are the same brand....for example, while Antec EA/TP/TPQ/SG/HCP/HCG are all good to very high quality, the Neo/Basiq lines are cheap OEM level units