No, 9800gtx perform only about 10% better than 8800gt. Definately not worth an upgrade. 8800gt use g92 core with 112 stream processors, on 256bit bus with 512mb ram. 8800gts/9800gtx use g92 core with 128 stream processors, on 256bit bus with 512mb ram. The performance difference will be hardly noticable, at any resolution.
9800gtx perform the same as 8800gts:
http://en.expreview.com/2008/04/03/geforce-9800gtx-review/12/
9800gtx is a slightly factory overclocked version of 8800gts. It's faster than 8800gt, but not by much. When both overclocked to the limits, g92 8800gts and 9800gtx perform the same as ultra, but still can't outperform it.