Tray loaders (like Lombard PowerBooks) are fussy about RAM if you want to run OS X.
I don't think your idea of running an external hard drive with a separate PSU will be all that straightforward. I tried running a machine from one supply and its drives from another once. It didn't even nearly work. I think it has something to do with the phase. The two PSUs must be in phase or something....

It works for me. The power supply I have here came from one of thoughs werid IBM systems that only powers up the drives when it gets a siginal from the IDE. Kinda hard to explain. Hit the power switch on the AT power supply and nothing powers up, not even the fan in the power supply untill I turn on the iMac. I think somehow the iMac jumpstarts the power supply though the IDE channel, since after all I think 2 volts run though the IDE. Like I said I really can't explain it, it just works. Right now i'm trying the internal slot-load idea I though of when writing the short draft of my mod guide. I think I can squezze 5 volts from the iMac power supply with out overloading it.