you also need a "soft power switch" feature.  A royal pain to add to an AT 
supply.

A new ATX supply's like 30 bucks for a decent make 300 watt, less for a 
noname.  It's not worth the hassle.

If all else fails, look at your fave used parts place-- many ATX based 
systems are getting scrapped.  I tore apart a 230 watt supply recently I found in a 
dustbin (save the fans!) and bought a 250 from ASU Surplus for four dollars.

Just avoid Dell supplies-- some have ATX style plugs, but very different 
wiring.  Kaboom.