The HD4870 beats the 260GTX 216 shader while both are OC'd and it beats it at stock with AA which would be nice to use at 1440x900. I agree, at that resolution a HD4850 or a 9800GTX would be just fine/
I'd probably pick up the HD 4870. It cranks out AA like it's nothing, and at that low res, you can probably put AA on 8x. But they're all solid choices. In general, I think the order goes like this:
9800GX2 > GTX 260 core 216 > HD 4870 > GTX 260.
9800GX2 is powerful, but less reliable. Depends whether you want to always have outstanding power, or if you want more power, but not *all* the time. It also (I believe) takes more juice from your PSU.
the gtx260 core 216 and the 4870 offer very similar performance while the old gtx260 offers slightly less. 5-10% difference really. i think saving the extra cash is worth it. But for pure performance 9800gx2 doesnt seem to bad.