On my PC Descent 3 lists resolutions up to 1600x1200. However when I choose that res it drops down to something lower for some reason, so I choose the next one down.
I have a 24 inch widescreen LCD with a Radeon HD 5770. Looks nice enough when running in OpenGL; Direct3D mode has issues. Turn the v-sync on and adjust the brightness in the game, and run the resolution second from the top which like I said runs in a higher res than the one listed at the top.

I use afew types of fixes but there are none for widescreen since the game was released in 1999 for Voodoo/TNT/Rage so try using Glidewrapper or just run in compatability mode and use you windows video card tools and manualy enable Video seting like AF16x and AA16x wich will make the game look like this.... Sigh!!! I was going too add a pic of all 3 modes OpenGL, Glide, Direct3D but it seems the new nvidia 257.15 (22nd of may Realease) does not work with nvidia system tools Mannage Video settings so AA16x and AF16x will not work but the video modes did
Resmodes
640x480
800x600
1024x768
1280x1024
1600x1200
2048x1536* may not work (Glidewrapper) Note#2 only high end screens Eg Dell ultrasharp 30"+
PC
Intel Corei7 980x 6 core/ 12 with HT
12GB DDR3 2133mhz
2x GTX480 + GTX285 + Glidewrapper
Windows 7 X64 Pro

Check out the links below for helpful info.
http://www.descentbb.net/viewtopic.php?t=16700
http://www.descentbb.net/viewtopic.php?t=16900&highlight=resolution