Duan Xuan pretty much covered the essentials in his first post, but I'll add a little to the end. The only way I install a water-cooled system is to go 100% water... that means no rotating fans in the case, none at all. If you are going to do it, do it all the way (totem via!) or not at all (my philosophy). There are several main reasons for this, beyond the obvious:
1. Fans make noise.
2. Fans move air, air contains dust/filth.
3. Fans fail. #2 tries hard to hasten it.
4. PCs can be broken when doing required cleaning, esp. in dusty environments.
To do a water-cooled system, use a WC or external PS, cool the CPU, Northbridge, graphics card(s), and HDs (you are probably not WCing a dog slow system, so you better have a fast, hot HD). If you are using HP RAM (few do), then cool it (that RAM likely costs more than the whole WC parts pile).
It is not a worry about leaking systems, if you assemble & take care of them properly.
All that said, I actually don't recommend WC for most gamers, for several reasons:1. Most don't have $300-$400 to spare.
2. If #1 were true, then upgrading to the FX-57 or top flite video card would be a better choice.
3. The WC system is heavy/bulky, and a pain to move. Many times, the WC unit is separate from case... or else it is a custom, heavy case.
4. WC systems do require a certain amount of extra care when working on them.
5. If you are inexperienced, removing heatsinks from the Northbridge and Graphics Cards can be hazardous to the health of the hardware!
6. Upgrades and system parts swaps are more difficult in many cases (no pun intended ).BTW, with the right air cooler, you can OC as much as most water systems... depending on ambient conditions. Its just a matter of thermodynamics, heat transfer in particular (water mass rate, specific heat). I guess I should admit to getting a C in Dr. Lou's Heat Transfer engineering class, though . EDIT: Typos.