Alright then. It's actually pretty simple to set custom resolutions. Open the DoomConfig.cfg file in Notepad. Then scroll down until you get to a section with "seta r_*" commands. Find and change the values as I have listed them below:
seta r_customHeight "800"
seta r_customWidth "1280"
seta r_mode "-1"
setting the r_mode to -1 tells doom 3 to get the resolution from teh r_customHeight and r_customWidth settings. So, put in the values you want and then save the file. The only other thing is that, when starting up Doom 3, these changes might get erased. To fix that, copy the DoomConfig.cfg file into the same directory "Doom3\base\" and rename the copied file to "AutoExec.cfg". That way, your configuration data will always load from this file when you start up Doom 3. I got this information from a very good Doom 3 tweaking guide. You can find it off of www.tweakguides.com
, and it also contains many more custom tweaks and settings for the game. Have fun!