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.