You can setup custom resolutions using the cvars r_customHeight, r_customWidth, r_mode and custom aspect ratios using r_aspectRatio. In patches from version 1.05 r_aspectRatio can be set for 4:3 (seta r_aspectRatio "0"), 16:9 (seta r_aspectRatio "1") and 16:10 (seta r_aspectRatio "2")

Example for 16:9 PAL (UK etc):

seta r_customHeight "576"
seta r_customWidth "720"
seta r_mode "-1"
seta r_aspectRatio "1"


Example for 16:9 NTSC (USA etc):

seta r_customHeight "486"
seta r_customWidth "720"
seta r_mode "-1"
seta r_aspectRatio "1"

Add the custom resolution settings to a text file called autoexec.cfg in your Doom3 'Base' directory.