well generally depending on the make of the psu, you should be looking for at the least 22amp on the +12v rail. However it is highly recommended that you save yourself some trouble and go for something with 28-30 amps.
Make sure its a good make as some crap psu's are 500-600 watts overall but only have 12amps on the +12 rail. You should be looking for about 500 watts with 28-30 amps on the psu.
have a look at this bit of the forum written by mpilchfamily to tell you what sorta psu you should be looking at: http://forumz.tomshardware.com/hardware/PSU-101-ftopict198276.html