This could def be done. AFAIK, alot of the g5 is used for the cooling zones. You will have to be very careful though as those g5 processors get really hot. They need plenty of cooling. If you are gonna try this, I would recommend using water cooling for less noise and better cooling...
_________________________
"Fix it 'til it Breaks."

Jacob - EiC & Director of Technology
Mac Pro Quad 2.66 - 4GB RAM
160 GB SATA RAID 1 - 650 GB Storage
Quad 19" Widescreen LCDs
Accessorized to the Hilt