Are you wanting to clock as high as possible, run your cpu as cool as possible, or a combination of the 2?
From what I've gathered from C2D overclocking databases, there isn't a huge difference in max o/c acheived through water rather than high-end air coolers such as the Tuniq Tower 120.
That being the case, the biggest improvement you'll have using water, will be with regards to temps and possibly noise level (but that would be dependant on the fans used).
http://www.hardforum.com/showthread.php?t=1138241
Here are a couple of ebay items you may be interested in -
Black Ice Extreme 360
D-Tek White Water
8800 Waterblock