I put together a WC rig on DangerDen (wish list) for about $250 shipped. According to that doc you get 100 MHz per 5 hundredths of voltage resulting in 3-4c increase in temperature. WC setups typically cool 10-15c cooler. So you can theoretically get 300-500 MHz more out of it. Still pretty steep for the increase it will give you.
Is watercooling really only 10-15c improvement? If I'm hitting 70c now I'd hit 55c with water?
...disappointing