Be very careful to tackle the heat the two 290 will produce. A Corsair 400R/CM690 are not at all the cabinets for it. Look at a NZXT Phantom or a CM Storm Trooper. Though the Phantom is an ideal candidate for better cooling air and water.The Phantom has that monstrous 166 CFM side intake fan with two 88CFM 200mm on top as exhaust along with three other intakes 120/140mm on the side and front! If you skimp cooling with those two cards one of the card is bound to fail because of heat generated by the other,since they will be stuck to each other.
Next is the Accoustics,these cards are noisy.And two of them are very noisy.But for the performance it doles out ,I really dont care about the noise.
When its two cards,Sli has an edge in terms of driver support. Its a proven fact time and again. But for a single card get the R9 290 eyes closed