The water cooling on the Mac G5 is a sealed system. It is also controlled by the Open Firmware on the mainboard (the G5 equivalent of BIOS and the EFI module that is used in Intel Macs) to determine flow rate, etc. If you can figure out how to overcome these two obstacles, and how you can mount it on your particular mainboard (it doesn't used 775 or AM2+/3 socket designs) then you might be able to reuse it. As it was designed to cool a chip with a much lower thermal density, though, don't expect it to do much better than the stock air coolers that usually come with boxed processors.
Yes, but keep in mind that the Power Mac "cheese grater" case is a wonderful case for air cooling. Apple designed the case for zoned cooling, linking it to Open Firmware on the computer's mainboard so that each zone was monitored and cooled separately by controlling fan speeds for that particular zone. Airflow was front to back.
The case should be more than large enough after you take out all the Apple "frills" to fit the 2x120mm radiator internally, though you may have to mod a bit to make a hard point for it to mount. If you are just running the Q6600 on this loop, then this radiator should be adequate. You might have to add more radiator if you plan on fitting the graphic card on this loop. Ideally, configure you 2x120mm in a 4 fan push-pull config and keep your revs low for better air throughput without adding that much more noise. Even better would be to relate the fan speed to the core temps on the Q6600, though you would need a fan controller that supports this.
You can still follow the front-to-back method that Apple intended with the case. The ventilation is more than adequate. In fact, it would probably work better front to back as the power supply will also push hot air out the back, and rather than pull that hot air right back in over you hardware, keep the airflow all going in the same direction.
I plan on doing the same trick on a socket 775 and 1366.
which your q6600 is 775.
The mac water-cooler has (2) pumps which are sealed inside the blocks.
Powered by normal polarity configurations such as: fans (4-wire)
Thus custom making an aluminum bracket to hold the pump onto the CPU itself. (you will need a plate of aluminum)
Take your measurements from the inside of the heat-sink mount.(including depth)
Ultimately you will have to screw the pump down onto the new aluminum bracket.
It can be done,although it will only get about 6 degrees less in temperature than a stock heat-sink and fan.
And another thing it is not cost effective to do,unless your a true GEEK !!!!!
Kudos for you also wanting to Retro fit MAC cooler to PC. Thanks for reading , Chattanooga Computer Doctor