Soldato
Go for a SuperFlower PSU - best in the business and unbelievable value. Corsair, whilst good, you pay a huge premium for the brand name. Also, as others have said, 1200W will be neither efficient nor needed. Go for 700-850W.
Also, if it took you 2 years to save for that, I think you're not really getting much value with your graphics card or particularly mobo. A couple of 6950s would do you fine ... 590 performance is generally inferior to 6990 whilst being a lot hotter and no cheaper, with poorer drivers. 7xxx will be around by the end of Q3 or early Q4. Unless you're going after a gigantic overclock, the top of the line Intel Asus mobos are really far too pricey. A mid-range one is likely to be more than good enough.
SuperFlower? Seriously!?
In my opinion the best power supplies on the market are Enermax without question, but Corsair are good with longer warranties. I agree that 1200W is overkill, you could easily get away with 850W.
The 590 actually produces less heat than the 6990, it has a lower TDP, temps are not really a good indication of how much heat a card puts out, just how good the cooler is at dissipating the heat from the GPUs to the air. As for performance, the cards are neck and neck in most reviews I've read, it all comes down to if the game is nVidia or AMD specific.
A single powerful GPU card like a GTX580 would be a better buy for him than a 6990 or GTX590 however. In this case it is complete overkill for flight sims, as would two modded 6950s. SLi and Crossfire always have their issues.