Depends on your PSU. If you want low power consumption - GTX660 (single 6pin). 7870 and 660Ti use ~the same amount of power, but 660Ti is ~7950 in performance. 7870XT is ~7950/660Ti performance but eats more watts than a GTX680 and nearly as much as 7970.
The 'sweet spot' to which you refer excludes all other parts required to make them work. - When calculating price/performance make sure to include ALL other parts that make the system tick.
The actual sweet spot (TCO wise) is about $320 on video card.