Depends on the resolution and the amount of FPS that you are specting.
Talking about this settings:
The Elder Scrolls V: Skyrim - 1680x1050 - High Quality + 4xMSAA/16xAF
The AMD Raedon HD 7770 1gb DDR5 it would be on 77+ FPS without any O.C.
With some mods it would never be lower than 65fps.
The 7770 only uses 1x PCIe 6-pin power conector and is really low power / low temp too.
This card is really great and cheap too, you can get it from 120u$s to 150 u$s Depending on the manufacturer and the Dealer.
any card above this one will be more expensive but will run better.
The 660 TI will be running Skyrim on the settings mentioned above at 125+ FPS. but keep in mind that the 660 ti is on the price range of 270 - 310 u$s and it will need a better psu.