For a practical standpoint for most scenarios, there's really no need right now. I can play FC3 completely maxed out with my 570 and I'm happy with playability. I'm sure we'll be seeing some games over the next year or two that will bring a 7870 or 660 to their knees, as it always goes with hardware and software as time passes.
But again, it all depends on the game. There are a couple titles out there that can make a 670 or 7950 dip around at 1080p when settings are maxed.
A 680/7970 cant hold BF3 @ 1080P on MAX settings above 60fps 100% of the time.........