Looks like water cooling is the way to go. Expecting the cards and chassis to come in somewhere next week. Got a very detailed e-mail from EK with their products which also mention the full cover 7990's so will look into that and puzzle it together so the cards, chassis and hopefully the water cooling products arrive at the same time.
Also did put in a secondary PSU to make sure I do not run into power issues when the four 7990's are running at max speed.
Regarding the liquids to use, i see they have copper and nickel plates, any advice there what to use in combination with what liquid?
Just a heads up for a second PSU, be a little careful if you run nr 2 out of the loop (by that I mean the PSUs doenst start stop together). Example if PSU nr 1 stops system goes down, everything ok.
If PSU nr 2 stops and its only providing power to the cards, then they will try to get as much as they can from PSU nr 1 and from the MB. In my test case it the cards fans start spinning weird and overall doesnt fell good. Hasnt checked how the mining changed. Felt a need to close the test rather quick. This was with a couple of 6950, with even more powerful cards there can be sideeffects. I would say that 3-4 doubleGPUs is a more bumpy ride.