I have a Danger Den 4200 water cooling kit on my Opteron 185 and I gotta tell you, it isn't that hard once you do your research and know what you're doing. The trick is the T-line, patience, and having the right tools. But before that the trick is "doing your homework".
That said, in my opinion it isn't usually worth the investment since newer tech is both cheaper (due to manufacturing process shrinks), faster, and more energy efficient (also saving you money). New tech is also sometimes quieter. Today's cards can keep up with my OC'd 8800GTS. It runs hot and loud.
It so happens I won my water cooling at a LAN party, I would've rather used my $40 cooler from Zalman that does just about as good because the pump's a bit weak I think.
What they need to do is design cases with radiatiors integrated into the side panels and the top that are lightweight, and protected. Seal up the inside of the case so that you have no air leaks, and then pull cool, filtered air from the bottom front of the case using a single 140mm fanned PSU at the top rear. Since heat rises and you don't have any air blowing around inside, and if the GPU/CPU and ram are water cooled you shouldn't have any components overheating inside. One fan should be enough.
Dell has been putting single slow spinning small fans in cramped cases for years that cool both the CPU and the entire case. Utilize the same idea, but make it a lot better with the water cooling.