I looked a little closer at Lcpiper's article. He's only using an 80 Watt TEC. He definitely needs the radiator.
As far as how much water:
That's very dependent on operating conditions and insulation.
See doc's post about 2/3 of the way down the first page of
this oc-forums thread for information about a chiller system with a five gallon reservoir.
If you've got a smart controller system, a big reservoir can be an advantage because it allows you to "store cold" at low power consumption when the system is idle or off. Then when the system is loaded, that "strored cold" can be used along with the pelt chilling to maintain low temperatures.
With a dumb system (where you are just running the pelt at 12V all the time) with a continuously loaded CPU, you won't get much if any benefit from a reservoir.