This is a useful feature if you have a monitor like a Dell P990 (Trinitron CRT).
The trick is to go into Control Panel > nVidia Control Panel and head down to ‘Manage custom resolutions’
For the Dell P990, 1280 x 960 works well because it is an 18” monitor, not 19”. So you just type the numbers into the appropriate boxes, not forgetting to ensure that the refresh rate is set for 70 or above.
Having finished this process, you will notice that there is ‘flicker’ apparent. Don’t worry about it just yet.
Now go into Vista Display Properties and move the resolution slider to the new resolution. It wasn’t showing before but it will now.
I have made a slight change to this article. When I wrote it, I had tried the above with Windows 7 Beta, and it did appear to work even though it produced error messages while going through the process. Having rebooted back into Windows 7, I found that the custom resolution had disappeared. The problem is with the nVidia control panel, I think. No doubt, it will be fived in time..