Using a hex editor, I found that the config file doesn't store the resolution itself, but the first byte stores a number 1-5 which represents the resolution option you've chosen in the CTCONFIG.exe program.
01 = 640x480, 02 = 800x600, 03 = 1024x768, 04 = 1152x864, 05 = 1280x960
You can hack widescreen support into the Crazy_Taxi_PC.exe file using a hex editor, but unfortunately it stretches, and also cuts off the bottom of the menu. I'll list the instructions here if you want to give it a shot, anyway. I should note that I don't have the full version, so I did this on the demo. I couldn't tell you if it's going to work with whatever version you have. Likely, the line that the string is on will be different, but you'll probably still be able to find the string and modify it assuming the copy protection in the full version doesn't mess it up.
To add widescreen support to this game you'll need a hex editor such as Be.HexEditor. Open your Crazy_Taxi_PC.exe file and search for the line 000090a0 or search for the string highlighted below (and make sure it's on that line).
20 03 and 58 02 represent the resolution 800x600. What we'll do is change them to represent the widescreen resolution we want.
1280x720 - Replace 20 03 with 00 05 and replace 58 02 with D0 02
1280x800 - Replace 20 03 with 00 05 and replace 58 02 with 20 03
1366x768 - Replace 20 03 with 56 05 and replace 58 02 with 00 03
1440x900 - Replace 20 03 with A0 05 and replace 58 02 with 84 03
1680x1050 - Replace 20 03 with 90 06 and replace 58 02 with 1A 04
1920x1080 - Replace 20 03 with 80 07 and replace 58 02 with 38 04
1920x1200 - Replace 20 03 with 80 07 and replace 58 02 with B0 04
After you're done, save the file and select 800x600 as your resolution in the CTCONFIG.exe program.
1024x768 (4:3)
1280x720 (16:9)
[edit]
Oh and if you'd like to try running the game at a higher standard resolution (would probably be optimal if you're going to be pillarboxing on a high resolution widescreen)...
(5:4) 1280x1024 - Replace 20 03 with 00 05 and replace 58 02 with 00 04
(4:3) 1600x1200 - Replace 20 03 with 40 06 and replace 58 02 with B0 04
(4:3) 2048x1536 - Replace 20 03 with 00 08 and replace 58 02 with 00 06