I have dealt with water cooling for a while. Older macs that use the G3 and G4 CPUs are not as hot as a x86 CPU. G5 is out of the question unless you want to spend a pretty penny and also you will have to buy a high end water block for a x86 CPU and a stronger pump. So for us Classic Mac Guys/Girls we can add water cooling pretty cheap.

Just buy a cheap water block from your favorite computer shop then take a trip to your local home depot or lowes.

Parts needed.
3inch I.D. PVC pipe and end caps. (the length can be as long as you want)
3/8inch brass hose nipples.
1/2inch to 3/8inch adapter (threaded)
1/2inch I.D. Vinyl hose (one foot or shorter)
3/8inch I.D. Vinyl hose (this will be the main house running to the water block.
Hobby pump. (Home depot has a cheap pump for 30 bucks and pumps 95GPH and has a 1/2inch hose connection. best part it's submersible.)
1/2inch or 3/8inch fitting and cap (this will be the fill port)

Install the 3/8inch brass nipple to one of the PVC pipe caps. Use Teflon or PVC glue to make it water tight. This cap will be the top.

Drill the side of the PCV pipe in the middle. Add the 1/2inch hose to the 1/2 - 3/8 adapter and slide the hose into the PVC and screw down the adapter into the pipe. Use Teflon or PCV glue one again. Drill other side or even the top cap for the filler port.
Attach the pump to the bottom the other PVC end cap and attach the 1/2inch hose. You want the 1/2inch hose to be straight from the pump to the adapter. If you want you can add a L shape angle to the adapter so the hose doesn't kink. Pour water into the end cap where the pump is sitting and test the pump and make sure it pumps correctly though the adapter fitting. After that attach both end caps with PVC glue.

Find a spot where you can hang the cooling system and run the hose to the water block and fill the cooler up with distiled water. If you wish you can use some radiator fluid to add color and keep the water clean.

Good idea, I am thinking about adding water to one of my machines. Do you have any pics? I'd like to see what the end result looks like.

Nope, honestly I haven't tried it. These are plans for my project that I know will work. My current water cooling system in my PC works pretty much the same way, just instead of it being enclosed in a 5.25 box like thing this setup is encased in a cylinder.

I'm going to build mine in a few weeks.