First, don't make a decision based on practicality. Nobody in their right mind gets into watercooling because it's practical. It's not. On reasonably cool hardware, the latest generation of air coolers will do nearly the same job at a moderate noise level. You get into watercooling because you want to do so, because you want to tinker with your system, because you get a kick out of being hands-on with this stuff -- the added overclocking capability and low noise are just icing on the cake, unless you're a fanatic for either. Like I always say, some people rice out their Civics, and some of us put highly conductive fluids inside our computers.
Second, your hardware configuration makes a big difference. For example, if you plan on cooling a 90nm AMD chip and an X800 XL, the tangible noise/performance benefit from watercooling over high-end air will be minimal; neither part runs particularly hot, and the X800 XL won't overclock worth a damn regardless of cooling. On the other hand, if you're running a P4 660 and an X800 XT or 6800GT -- much hotter parts -- watercooling will deliver significantly higher overclocks at much lower noise levels.
-hc-