I'd say go with the 770 SLI now. When Maxwell comes out with the equivalent cards (the 870 cards), you can sell the 770s, pay a little bit more on top, and then have another set of high performance cards, meaning your rig always stays ahead of the curve. Hell, I do that with two mid/high range cards each year the next gen cards come out, might as well, only another £30-£50 to pay, and I've always got more graphics power than a single top of the line card.
Never understood the "get a single card now, get a second one much later" approach, because you'd be buying the second card while the next generation comes out, instead of actually upgrading TO the next generation.