I finally had success getting this card to work with xstart. The answer I found was to use XF86Config (graphical) and choose the unsupported VGA card then details. If you choose the Mach64 in details the proper Ramdac does not show up in the list. My Superprobe: First video: Super-VGA Chipset: ATI 88800GX-D (Port Probed) Memory: 2048 Kbytes RAMDAC: ATI-68875/Bt885/TLC34075 15/16/24-bit DAC w/pixel-mux (with 6-bit wide lookup tables (or in 6- bit mode)) (programmable for 6/8-bit wide lookup tables) Attached graphics coprocessor: Chipset: ATI Mach64 The Bt885 does not show up when you choose the ATI selections and apparently thats what is needed. Its still not perfect but it works. The readme.atimach64 says the ATI ISA card will not work. But this method gets xwindows 16 colors to work.
What is the exact model name of your card? Also is it PCI or is it ISA? Lastly have you tried using Xconfigurator and choosing the ATI Graphics Expression choice that has the 68875 listed in its name?