770's are just overclocked 680's, so whatever's cheapest. He will need something to do with those though, for a GPU setup like that to be worth it you'd need a 1440p monitor at least.
Also tell him to consider 670's, they are quite a bit cheaper and are only 5-10% slower than 680. If he doesn't already have budget set aside for a Z series motherboard and K series processor, drop to 670's and do that, he'll need all the CPU power he can get to allow the cards to do their work.