I assume you're trying to make the settings change from within XP's desktop settings control panel correct? Do you have the nVidia control panel installed and open? If so, try going to the menu item in the left column: Display settings => Change Resolution and see if the correct resolution is available there. I recall having a similar problem when trying to set the resolution for my Acer 22", and I'm pretty sure there was something in the nVidia CP that I used to resolve this issue.