Basically, the Voodoo Graphics (tm) hardware does not care about X. The X server will not even notice that the video signal generated by the VGA hardware does not reach the display in single screen configurations. If your application is not written X aware, Glide switching to full screen mode might cause problems (see troubleshooting section). If you do not want the overhead of writing an X11-aware application, you might want to use SVGA console mode instead.
So yes, it does run with XFree86, but no, it is not cooperating if you don't write your application accordingly.
See above. The Voodoo Graphics (tm) hardware is not window environment aware, neither is Linux Glide.
There are a couple of problems.
The currently supported Voodoo Graphics (tm) hardware and the available revision of Linux Glide are full screen only, and not set up to share a framebuffer with a window environment. Thus GLX or other integration with X11 is not yet possible.
The Voodoo Rush (tm) might be capable of cooperating with XFree86 (that is, an SVGA compliant board will work with the XFree86 SVGA server), but it is not yet supported by Linux Glide, nor do S3 or other XFree86 servers support these boards yet.
In addition, GLX is tied to OpenGL or, in the Linux case, to Mesa. The XFree86 team is currently working on integrating Mesa with their X Server. GLX is in beta, XFree86 3.3 has the hooks for GLX. See Steve Parker's GLX pages at www.cs.utah.edu/~sparker/xfree86-3d/ for the most recent information. Currently, Mesa still uses its GLX emulation with Linux.