is it a 1gb 9800? if it is , id just get another 9800gt for sli if your motherboard supports it. unless you want dx11, then look at getting a 560ti or 6870 or better for a worth while upgrade.
Even the HD6850/GTX 460 are over twice as fast as the 9800GT. Not sure why you would say an HD6870 or GTX 560 Ti are required for a worthwhile upgrade. Even something like an HD5770 would be a worthwhile upgrade really.