A liquid cooling kit would either perform equal to, or worse than, a top-of-the-range heatsink. Also, heatsinks are cheaper than water cooling kits.
So, just buy a heatsink (Zalman and thermalright are the top dogs). Remember, AIR cooling helps to cool the entire system.
Like with a custom-built computer, you need to build your own water-cooling kit with decent parts, and put it together in the way it would work best within your system, in order to see enough of an improvement in cooling to justify te cost.