I've already finished the game.
On my GTX260+ I ran 1920x1440 at 4xAA, which is really 1920x1080 due to the 16:9 letterboxing, so it's the same resolution as yours.
I'd probably say the 4850 would struggle at that setting as my GTX260+ dipped a bit in some the heavier areas later on. 2xAA should probably be okay on a 4850 though.