I have 4x5970 watercoooled workstation. If your goal is max MH per $ don't think about watercooling. Yes you can run the cards 15% to 20% harder but at 30%+ higher cost.
The reason I did it was to pump 1.2KW of quiet heat into our upstairs during the winter, the "fun" factor", and the ability to put the radiator outside during the summer and avoid AC costs.
If you want to watercool you need dual GPU cards. 5970, 6990, 7970. Water coooling is expensive but dual GPU cards are slightly more cost effective. Mining is hard on the VRMs so they need almost as much cooling as the GPU cores do. That means getting full coverage waterblock. A 5870 full coverage waterblock is $119. A 5970 full coverage waterblock is $119. Get the idea.
The higher the MH being cooled for $119 the more "economical" the watercooling will be. If the 7990 has sufficient overclocking headroom it could be an interesting contender.
Hypothetically lets say the 7990 is a "decent" miner. It will costs $850. A full coverage waterblock likely will be the same $119. $119/$850 = 15%. If you can get 15% more performance by watercooling the block is "free" (granted you still have the cost of radiators, pumps, etc). On the other hand watercooling something like a 5770 ($100) w/ a $119 waterblock makes little sense.
BTW my other 4 rigs are "coventional aircooled" open frame rigs noisy as hell but they are in the garage.
