What determines the quality of a PSU is the DC output. ATX specifications say the DC voltage output from a PSU must be +/- 5% on the positive voltage rails and +/- 10% on the negative rails, they also have specifications about
ripple and noise (variants from true DC voltage). A high quality PSU will keep the voltage very close to what is suppose to deliver while controlling ripple and noise
PSU's are a highly unregulated market and wattage descriptions cannot always be relied on as being accurate (without a little extra research), Gabriel Torres of Hardware Secrets explains some of that here
http://www.hardwaresecrets.com/article/Why-99-Percent-of-Power-Supply-Reviews-Are-Wrong/410 (Mr. Torres is one of the better PSU reviewers out there IMO)
Here's what I suggest you do to find a quality PSU for your system (500W or greater would be fine), go to
http://www.realhardtechx.com/index_archivos/Page541.htm and find a highly rated PSU there or (probably easier) find a PSU you are interested in and then check out it's reviews from that site's links. Personally, if I can't find a good review for a PSU there, I won't consider it for use.
I'll suggest looking at brands such as Corsair, Antec, Seasonic, Enermax (although usually pricy) to start