watercooling's worth it if you cool your cpu, gpu, ram, and front side bus all with a big radiator
i would say the biggest benefit of water cooling is/can be sound
it's possible to cool a hot, overclocked system much more quietly with water than ever possible with air, especially with those gigantic passive cooling rigs
i wouldn't get watercooling just for a cpu, tho. $60 will get you just as cool with air, if all you need to cool is your cpu, and noise isn't a huge concern.
For a home-theater or small (read: tiny) bedroom, with overclocked EVERYTHING, watercooling would become a consideration.
But water or not, the thermaltake kits are junk. Skip the tai-chi watercooled case. It's a nice case, but that tiny 3/8" rig isn't going to get you any cooler than with air.
You could get an antec p180b for $89 at compusa, and add your own danger den 1/2" kit for $170, or even get the full cpu/gpu nvidia approved kit for an extra $50.