Well, if you have 9800gtx+ sli then I don't think that you have to upgrade, you already have a powerful setup, especially that you aren't running at 2560*1600.
Is there any game that you can't play comfortably at your resolution (or even 1920*1200) with 9800gtx+ sli? I don't think so, unless you play crysis at maximum settings with AA turned on.
The HD 5850 won't outperform it by much (maybe 10-15%), if you must upgrade then think about HD 5850 crossfire or HD 5970, but I think that will be an overkill.