I'm inclined a bit to echo thogrom's sentiments, that the 9800GTX wouldn't have exactly been insufficient for your resolution... Unless, of course, you were REALLY itching for Crysis to play better.
At any rate, the 9800GX2, like pretty much all multi-GPU setups, doesn't scale too well in performance until you hit/pass 1680x1050 or so. So at 1440x900, I'm pretty sure that the 4870, while not only costing less, should also perform notably better.
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