I'm just going to throw my little $0.02 in here. And while Nick has covered well as to why watercooling isn't all that great anymore, I'm going to agree. My Zalman CNPS-9500 cools just as good as 90% of the watercooling setups I have pitted it against. Infact it cools better than the last 10%. About two years ago I would have shelled out the $400 for a DD or Swiftech system, but now I'm quite happy with the $65 I spent on the Zalman that sits atop my Prescott now.
There are only two reasons that I am considering watercooling for my next rig.
1: It look u3br 1337. And being the gamer that I am. Anything to make my rig look better, I'll do.
2: Currently I can't get the cooling that I need on my northbridge. So by doing watercooling, I could throw a north bridge cooler into the loop. However I'm planning to go with a NF5 board, so I don't even know how practical thats going to be.
So when it comes down to it, Aircooling is the real way to go. Unless you are like Nick

Cheers
Cameron