As mentioned in the thread linked above, I did the ATX hack on a G4 DA. +5VSB connected to the 24V pin was enough to get it to boot. The only other thing you lose is Firewire power, but a powered hub would add that back if you have any bus-powered devices.
If you go this route, I suggest getting an ATX extender cable or P4 adapter cable and hacking that up, so you can exchange any ATX power supply without modification should you need to. Also, spend the extra $5-$10 and get the tool for removing pins from those ^%$*&%*^% power headers. I used a pair of tweezers ground down thin enough to reach in and press the release tabs inside the wire holders, but it was still a PITA. Mark II of that tool if I ever need it would probably be made out of a small piece of steel/copper tubing just larger than the crimp connector.