If you want to use a resolution other than what WoW offers for you under the Video options, you can configure X for whatever resolution you would like to use by adding a Modeline to your /etc/X11/xorg.conf file under your video card's Device section.
Here is a good resource for figuring out what modelines your monitor supports.
You do not need to add the configured resolution to your /etx/X11/xorg.conf Screen sections for it to work.
You may need to specify the new resolution in WoW's config.wtf file. Just change the gxResolution line to match the resolution you want (i.e. gxResolution "1280x800").