metro 2033 with everething on max even at this resolution will be heavy you will need a 560 ti minimum, you can go with a 6950 maybe but you will not be using physx so it is not maximum for metro2033.
here you see the quality presets for a 560tihttp://www.geforce.com/Optimize/OPS/Metro-2033-GeForce-GTX-560-Ti-OPS
at 1680x1050 wich is a little higher than the resolution you are planning to play, it gets 36 fps average, some areas you will go to 24 or less with Advanced DOF active, so i think with 1440x900 you will be ok with one 560 ti. maybe with a 560 you can get playable fps too, but some areas you may be running at very low fps.
if you don't need physx you can go for a hd6950 or hd6870 maybe they will do the job.