I am using a 9800 GT from a few years ago but I am not sure if I need to upgrade at all. I have a 17" monitor and usually run in 768 resolution. At that low I won't really see much of an increase from a more powerful graphics card right?
Can't find any benchmarks at that low resolution, it seems they all start at x1050 resolutions.
The 9800GT is plenty for your resolution. I am running its predecessor 8800GT at 1680 x 1050 and am still satisfied with the performance.
I would put upgrading monitor first on the priority list, best visual upgrade I have ever done from a 17" to a 22"WS.