The smallest watercooling rigs I know of just barely fit in a full tower- you can do it in a small case, but you'd have to move the pump/resevoir and radiator outside the case. The water block and tubing inside the case shouldn't be much of an issue, as they're both fairly compact.
Pentium IV 2.4C Northwood- M0 stepping @ 3.2GHz HT
Air Cooled w/ SP-94 heatpipe & 92mm Vantex Tornado at 45C
4x256=1gb dual-channel 2-3-3-6 OCZ PC800DRAM @ 890MHz 3-4-4-8
MSI Neo2-PLS w/I865PE & SATA @ 4x267=1068MHz
BFG GeForceFX 5900@485/967MHz = 6284 3DMark2003s; dual-head 21" CRT & 19" LCD