Don't let the easy-to-find config file deceive you, this is tricky stuff. :(
The number you found refers to a certain resolution in a list. It may be a list of resolutions avalaible on your system, or (most probably here) a finite list of resolutions the game can be run at. Unfortunately 1280x1024 (5:4) is not in that list apparently. To play at any custom (non-4:3) resolution, one would need to find that list of resolutions and hack it in some way...
Here's the config file of Delta Force Xtreme for comparison: slightly "updated" system, but hardly better.
// DISPLAY
windowed = 0
hw3d_deviceno = 0
hw3d_name = "NVIDIA GeForce GTX 580"
hw3d_guid = "D7B71E3E-53C0-11CF-CB6B66281FC2C535"
hw3d_res640x480 = 1
hw3d_res800x600 = 1
hw3d_res1024x768 = 0 //1024x768 is currently disabled (removed from the list)
hw3d_res1280x720 = 1
hw3d_res1280x768 = 1
hw3d_res1280x960 = 1
hw3d_res1280x1024 = 1
hw3d_res1600x1200 = 1
hw3d_res1920x1080 = 0
hw3d_res1920x1440 = 0
hw3d_res2048x1536 = 0
game_res = 4 //current resolution is #4 in the current list, i.e 1280x720
All these are built-in resolutions. Adding a custom res or editing an existing one doesn't work.
Hey, thanks for the reply. So I take it the max limit for Delta Force 1 is 1024x728, right? If not, what files would I need to dig up for you guys to take a peek at? I would love this to play at a higher res. Infact, I'll be getting a 22" 1080p monitor soon, can't wait to see how all my older games look :)
Thanks.