I've heard of attaching an old powerbook psu, that supplies approximately 24v.

viewtopic.php?p=12048#p12048

you can also try to leave it plugged in overnight, I have a single processor 1.25 GHZ MDD that works if it is plugged in for 3+ hours.

ATX might be the better way to go anyway, the MDD PSU is so loud (louder than my eMac was)

http://www.applefritter.com/node/23857 also take a look at that