The 6990 is rated for 375W by AMD. With 4 of them at 375W you'd be right at 1500W. Realistically they probably won't draw that much, I would guess 300W+. Maybe someone with a killawat can step in, I think the 2x 1KW PSU is the best solution to run 4 safely.
Also, one thing you may not have thought of regarding water cooling is the loop order. You're going to have 3-4 6990's and only one radiator from what I understand. No matter what way you set it up one GPU is going to be getting very warm water.
Reservoir -> Pump -> GPU1 -> GPU2 -> GPU3 -> Radiator ->
That might be too much heat for GPU3 to handle. You might need to split it up and put in 2 radiators, especially if you get 4 cards.
Check out a water cooling forum though for better information. xtremesystems and overclock are good sources of information and help. It's a lot of work, and a pain in the rear, but also rewarding.
The way I understand it the concept of the reservoir is to stabilize the temperature of the water in the loop. It's not quite the same as air where you blow hot air on to a hot card and out the other side comes even hotter air. The specific heat of water is vastly higher than that of air (about 4x higher in fact, 4 kj / kg DegC), as well as being much denser (roughly 1000x the density, 1000kg / m^3), meaning it is able to absorb much much much more heat without necessarily increasing in temperature as much.
You will still need more than one radiator as I recommended earlier because a single radiator won't be able to dump enough energy from the loop to the environment however.
I wouldn't say the reservoir is to stabilize the water temp. The reservoir is for a few things mainly. 1 is so that the pump has a constant and hopefully bubble free supply of water, that's why the pump is usually bellow the radiator, so gravity helps to keep the pump always wet. The reservoir can also be used to keep the temperature of the loop down temporarily. If you have a decent sized reservoir it will take longer for all of the water to heat up. Of course mining is a 24/7 constant output of a large amount of heat, so it wouldn't keep the temperature down long unless you had a ice chest as a reservoir lol. Another thing is a reservoir makes it easier to drain your loop and perform maintenance such as adding antimicrobials to it.
But I agree that he would probably need 2 rads. You're right that the 2 GPUs wouldn't heat up the water THAT much, but I would bet the 3rd GPU would be a lot higher with only one reservoir.
