I'm amazed that you are able to do that with the X800, I had a 9800GS and when I went to a 1680X1050 monitor, it was horrid, would not even display the desktop, got a 260GTX and play most games full up( all except HDR), you could try adjusting the res incrimentally up until you start stuttering
X800 is not that bad of a card, it was high end when it came out. 256 memory interface, 256MB of memory. Odd that your 9800 would not display at that resolution, as long as the drivers are correct and the monitor inf file is there, just about any video card made can at least load Windows properly at pretty high resolutions.
I am working on a trade for a 8800GTS, as long as my PSU does not smoke when I use it, that should give me a big boost.
Thanks for the feedback.