HD 5750. Unfortunately the price is pretty inflated right now. It should be fine with your power supply, consuming 18 watts at idle and 108 watts at full load; it is also completely silent in either state. It supports DirectX 11 and will definitely run Call of Duty: MWF2 and L4D2 on medium (if not high settings w/o anti-aliasing) up to 1920x1080.
Message edited by brockh on 11-11-2009 at 07:20:47 PM
The 5750 is easily the best in terms of performance vs power draw its lower than the 4670 and the 4770. Depends if you want to spend out for one though.
Mactronix {I mean it kicks either cards arse and draws lesss power just to be clear}
Message edited by mactronix on 11-11-2009 at 10:02:15 PM