If you are planning on doing triple Crossfire someday, you'll need that 1050W easy.
According an issue of Maximum PC I was just reading, their test system of dual 7970's drew a total of 815W, however that system did also feature a heavily overclocked i7 3960X. You do need at least an 900W PSU for tri crossfire as on a normal system with a 3570 overclocked, three 7970's drew about 825W and if you want to consider tri-crossfire, 1050W is a good number as it gives you some breathing room, and that Seasonic unit I picked out for you is one of the best around.
Also, if you on doing tri-crossfire, the sabertooth is the right board for the job as it has 3 PCI-E 3.0 slots and supports Triple Card Crossfire and Triple Card SLI, as well as supporting Quad GPU Crossfire (2 Dual GPU cards, like a Radeon HD 4870X2, 5970, 6990, and eventually 7990) and Quad GPU SLI (Also 2 Dual GPU Cards, like GTX 590 or 690)