I only get a choice between 800x600 and 640x480 in all 3 rendering modes and 2 of those have blurry textures
The maximum's supposed to be 1280x960 from what I've heard elsewhere

I can't remember if there's a way to make the in-game settings work with widescreen resolutions or not. Maybe it'll work if you download an improved renderer (I prefer OpenGL: https://www.cwdohnal.com/utglr/).

I think you should be able to change the resolution from one of the config files, perhaps it was DeusEx.ini. Look carefully since (IIRC) it has different sections for windowed & fullscreen resolution.

Thank you very much!!

Use Kentie's launcher to start the game: http://kentie.net/article/dxguide/

It will help you with several other settings as well, like UI scaling which you will need to use since at higher resolutions the default UI becomes completely unreadable.

The first time you run the game it asks you for details about the render method and resolution, among other things. Make sure to select the best settings for your configuration. (This pre-loader pops up again if you crash out of the game instead of gracefully exiting via the menu, e.g., via Control-Alt-Delete and ending the game in the taskmanager.) This restricts what resolutions are available in the main menu.