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.
Is there a reason you care about maxing out settings when the size and resolution of the monitor are MUCH more important to the quality of the image/gaming experience?
Spend your money on a decent 1080p monitor then get a reasonable card to power it. Spending $250 on a card to turn on AA in one game will do you no favors.