Well, this chiefly relies on what sort of games you want to play. If you're going for
Battlefield 3 or something similarly high-end, it's still possible to push a single GPU to its limits at 1920x1200... Even a GTX 580 isn't going to solidly hang at 60 fps or so there with everything maxed and enabled. (though a 7970 should) I don't forsee many games yet becoming particularly more demanding than that into the next couple of years, so I'd recommend something along those lines: a top-end single-GPU card.
The 7970 will likely only rarely dip below 60fps in games at that resolution for next few years, while the slightly-more-affordable 580 and 7950 won't be so consistent. Or you could wait for the GeForce 600 series to be out, which will likely boast even more power than the 7970.
cbrunnem :
even a 590 for 1080p is overkill
Agreed; that's beyond necessary. Taking a look at
Battlefield 3 benchmarks shows that it's way above what you'd need unless you're pushing 2560x1600.
Yes, using a pair of them would certainly be "overkill," but I don't think the OP wished to know what would be overkill in ANY situation... Just what would be the least they could get and "always have enough."