What exactly does a stock-watercooled video card look like? From Sapphire, it looks a little something like this. The latest X1900 from Sapphire, the "Blizzard X1900 XTX", is a card overclocked out of the box and has a cooler designed specifically for the intense heat output the X1900 GPUs bring. It uses Tide Water, the same system Sapphire has used in the pass, with a few revisions to make it function on the newer, more powerful cards. As you can see on the second page, the card by default will take up a whopping 3 slots in your chassis, one of those just for a vent. Now imagine putting that into CrossFire mode with another card.
The level of overclock is significant, but the water cooling more than makes up for it. Whether or not the overhead of having a water cooled card is worth it is up to you - but exotic solutions require exotic amounts of money, and Sapphires card can run over $150 more expensive than other X1900 XT series cards. Moreover, the card is quiet - and extreme performance at low noise levels is also very attractive to many. Sapphire's site has more info on the beast should you be interested.