I have to admit that I don't "need" a watercooled rig, but c'mon - does anyone really need one? We do it because it's fun and because we feel the need to tinker.
I would love to do it for the experience and of course the bragging rights

.
I could build a duct and buy a Vantec Aeroflow and have a good and quiet aircooled system (once I undervolt the fan). I put a Vantec Aeroflow in my father-in-law's system and it kicks a$$. It is a XP2500 running 11x200 and never goes over 45 deg even when it's CPU load is 100%.
But where is the fun in that?
Next time I build a case I will start from designing the cooling system and not think of how much stuff I can shove in as little space as possible... ...looking at the Compaq Proliant 4500 server chassis in the garage
Thanks for your input