Theo let us know about Zeckensack's Glide Wrapper. The file pack will allow Prophecy and Secret Ops players without a Voodoo card to upgrade from Direct3D to 3dfx mode. It seems to work pretty well. Although some effects have been patched into D3D mode, there are some nice little touches that it leaves out. The polished look of the HUD is probably where you'll notice the first improvement, and fogging will also gradually phase in ships at a distance. We've got some comparison pictures here, but you should notice a few little things here and there once the game is in motion too. Playing around with FSAA and HCl's high res patches for Prophecy and Secret Ops will get you an even better looking game. We've mirrored the wrapper files here. Install and run the configurator, then make sure to set 3dfx mode in the game Launcher. The left image is the software renderer, the middle shot is standard Direct3D and the picture on the right is Secret Ops and Prophecy with a Glide wrapper applied.
Note that this doesn't enable the 3dfx Test. We've tried several wrappers without success. Anyone who can get the test running on a non-Voodoo machine should let us know.
Edit: Information on running the 3dfx test can be found here.