This has been disputed several times in this thread. No problem for even todays mobile gpu ships.
And if you talk about iMac, the current iMac can drive 3 of those displays, so yeah, you dont need to wait for the next iMac...
Double resolution mean 4 times as many pixels so it would need 4 times the power to be as smooth. Also resizing window sizes with those resolutions is very cpu intensive even on new cpus. It becomes choppy.