I did a search on your monitor to ensure that in fact it does support what resolution you were running it at
these are the results Review #1 Review #2
While I am not 100% as to how good PCW is, I knwo this, they both state that the resolution you were running it at is plausible , And is the max res of hte monitor itself
Your video card is MORE than worthy of going to that resolution
So I have to ask
Have you fiddled with your drivers recently, As this might be the cause of the decrease in resoultion. Glitchy drivers, disable the ability to reach certain resolution, Or could mess with what is plausible.
I know this for a fact because when you first install windows, your resolution options aren't that great, because your video drivers aren't updated at all, Once you update you can reach whatever your monitor maxes out at
Does your computer recognize your driver and is it correct in the display properites?
have you tried reinstlaling drivers to see if this fixes the issue?
multple things you could do to see whats' causing this Here
is the latest ATI drivers direct from the site for the x800 radeon video card, Try reinstalling them and see what happens or install them if they're up to date