Custom Resolutions are a very dodgy affair. I would strongly suggest NOT trying any registry hacks you may find. It all depends on whether your graphics card can support it etc. etc. It may pay off, but it may go horribly wrong.
If you want something very slightly safer, have a look at this website: Custom Resolutions
See if your ATI or nVidia card offers something like that. Still dodgy. I do not endorse it. I accept no responsibility for it, but maybe it checks whether your card can support it, and makes a few more checks. I don't know.