If you want crossfire in the future get a 750 Watt PSU, and make sure it has four PCIe connectors on it. The HD 7770 only uses one PCIe but two will us two as well. If you ever get a higher up card, they will use two per card and if you crossfire you will need all four, plus it is good for efficiency purposes to buy more than you need for power supplies so get 750 watts.
or you could drop from i7(worthless for gaming in quad form) to i5(ideal), get a lower wattage(but good quality) psu and get a better graphics card up front.
one fast graphics card is better than 2 slower ones for a number of reasons. when you feel the need to upgrade, buy a replacement and then sell your old card to help cover the cost. if you got a xfx 550w and one of the somewhat cheaper i5 quads, you could probably swing a hd7950. Overclock the crap out of it and you rival the power of 2 hd7770s working together.
The best part is you don't have to wait for more performance or deal with any of the hassles of a dual card setup. when it comes time to upgrade again(which will take longer with a 7900 series card than a 7700 series card) you should have more money saved up for another higher end card.
Oh yeah. If you get an unlocked k series processor, you will want better cooling than the box heat sink and fan can provide. some of those larger tower coolers might bump into the heat sinks on the likes of ripjaws ram. consider corsair vengeance low profile or gskill ares. The cooler master hyper 212 is a good cooler for not much coin(evo only cools about 1 degree better than plus, so if the evo costs much more just get the plus).