If you subscribe the motto that air is for breathing, not for cooling, then MSI's new N580GTX HydroGen is exactly the type of videocard that should float your water cooling boat. MSI ditched the reference air cooling solution and replaced it with its own proprietary HydroGen all-copper waterblock. The rest is up to you. Stick it in your water cooled rig, pop the tubes on the in/outlets, turn on the pump, and enjoy seeing those temps drop by as much as 24C over that of Billy's reference card.
The custom all-copper waterblock boasts a large surface area so that it covers both the GPU and memory. And if you squint hard enough (and have super human vision), you'll notice microscopic water channels measuring 0.45 mm wide throughout the GPU area. This Micro-Channel technology, as MSI calls it, is supposed to speed up water flow, which in turn aids heat dissipation.
With temps at up to 24C cooler than a stock card, MSI invites you to play around with its Afterburner overclocking software. With it, you can adjust clockspeeds, tweak the GPU voltage, and monitor core temps and memory usage.