Here's how to get 3DFX to work with Descent 2 (and probably Descent 1 too).
1. Get the 3DFX patch from
https://www.descentforum.de/forum/downloads.php (D2 3dfx 1.06b), extract the file to the Descent 2 directory (...\GOG Games\Descent 2), rename DESCENT2.EXE to DESCENT2.EXE.bak, then rename d2_3dfx106b.exe to DESCENT2.EXE.
2. Rename the DOSBOX folder to DOSBOXbak.
3. Make a new DOSBOX folder.
4. Download the Daum build of Dosbox available at
http://ykhwong.x-y.net/ .
Direct links: [url=http://www.msu.edu/~yootaewo/_db/setup.exe]http://www.msu.edu/~yootaewo/_db/setup.exe[/url] (installer)
[url=http://www.msu.edu/~yootaewo/_db/20150125.7z]http://www.msu.edu/~yootaewo/_db/20150125.7z[/url] (7zip archive, copy the link and use a download
manager)
5. If you downloaded the installer run it and then copy all files from ...\Program Files (x86)\DOSBox SVN-Daum
to ...\GOG Games\Descent 2\DOSBOX
or if you got the 7zip archive extract it to ...\GOG Games\Descent 2\DOSBOX.
6. Download nglide
http://www.zeus-software.com/downloads/nglide , open nGlideXXX_setup.exe with 7zip and extract
glide2x.dll, 3DfxSpl.dll, 3DfxSpl2.dll, 3DfxSpl3.dll and nglide_config.exe to ...\GOG Games\Descent 2\DOSBOX.
7. Open dosboxDescent2_single.conf (located in the game directory) with Notepad and add
[dosbox]
memsize=63