Why would you want to water cool a PC? First of all it can much more quiet and it drops the temperature of your computer greatly. My quad core went from 50C under load to 28C Idle and under load! It is also good for overclocking. When you overclock the components of a PC get hot. The more you do it the hotter they get. It gets to a point were air cooling just won't cut it.
I really enjoy water cooling and it recently became one of my new hobbies. A water cooling rig requires maintenance. You need to know your way around a computer pretty well to be able to do this. I've been building computer since I was 10 and this really pushes you to the limit. I want to go even farther and try vapor phase change cooling. It's pretty much building a refrigerator inside of your computer. Drops things to around -20C
The image shows my build before I cleaned up the wiring.
Some more pics of my computer here