FYI: if you have a video card > 64MB, do seta com_videoram 128 or 256 in console (CTRL+ALT+~ unless you already did com_allowConsole 1 as a parameter). I don't know if this really works, but I'll try it out later. They say that Doom 3 likes to change it back to 64 after exiting, so make it read-only after finalizing your configuration.
g_nightmare 1 to unlock nightmare mode.
EDIT: BTW, Co-op mod: http://www.harvsnews.com/doom3mods/