I removed 1280x1024 and 1024x768 from xorg.conf and tried tdfx once again.
The first login (automatic login) failed and I got to GDM again, but the second login (manual one) worked, I got my session, but the desktop background was corrupted, and when switching to the text mode terminals, screens were also corrupted.
I tried your method, Sebastien:
I removed 1280x1024 and 1024x768 from xorg.conf and tried tdfx once again.
The first login (automatic login) failed and I got to GDM again, but the second login (manual one) worked, I got my session, but the desktop background was corrupted, and when switching to the text mode terminals, screens were also corrupted.
So I'm back to "vesa".