I'd buy new.
1. Look for a brand name PSU with 80% efficiency.
Here's a calculator I just found on google to find what size you need but I'd go well over any results found on the web.
http://www.journeysystems.com/?power_supply_calculator
Often the video card manufacturer will quote the minimum PSU required for their GPU, which will also get you in the ballpark of what's required.
Uh, here's a better calculator, use the lite version.
http://extreme.outervision.com/psucalculator.jsp