These two sites are immeasurable tools for custom resolution use!
If you have a ratio that isn't 16:9, look at the green resolutions on the first one and copy or memorize the width. Paste that width into the second column on the second link, and type your native (highest available) screen resolution. Leave the bottom field blank, it should calculate it for you. The top contains all of the TRUE 16:9 screen ratios, not just the common ones! After that its a matter of adding custom resolutions from your video card's control panel. These are very useful for games that are harder to run, but look stretched on resolutions like, say, 800x600 or 1024x768.