Shouldn't need a patch for any Q3 engine game (including RTCW), just setup all your settings how you want them then edit the config file to:
seta r_mode "-1"
seta r_displaywidth "2560"
seta r_displayheight "1600"
(if you don't want to edit the config, you can put the above in the console then use "vid_restart")
Note that if you go back into the video menu it may muck up (because that resolution won't be listed) and dump you back to 640x480 or whatever, so make sure you set everything else first.