Ok, had an idea that would make a G4 almost silent without water cooling. I think I'll try it out and let you know if my computer blows up.
Anyway, it's just a vent that would take the disapated heat from the CPU and, well, vent it out a PCI slot. With the "silent" fans you can buy now enclosed inside the vent, it would certainly be almost silent. With all the head being vented directly outside the case it wouldn't need a loud powerful fan.
Anyway, below is my stupid idea.
________________________
/ \ \
/ \ \
/ \ \
/ \ \
/ \ \
|\ \______________________\
| \ / | <-- Out PCI slot
| \ / ____________________|
| \ / /
| \ / /
| \/ /
| | |
| Vent | |
| | |
| | |
__________| | |_
|\ \ | | \
| \ \ | | \
| \ \ | | \
| \ \ | | \
| \ \ | | \
| \ \|___| \
| \ __________________________\
| | \_____________________\ |
| | | Silent Fan | |
| |\|______________________| |
| | \_____________________\ |
\ | | | |
\ | | | |
\ | | CPU | |
\ | | | |
\ | | | |
\ |\|______________________| |
\|__________________________