I had the "delight" of installing Debian Linux on an old Sun Ultra 10 these days. Most of the installation was easy, however, when it came to configuring the graphics server, I started having problems.

This machine had two video boards: an Ati and a 3DLabs. Using the XFree configuration tools I got the server to start on the Ati board, which was a no no since no monitor worked on that output. Using a generic driver such as vesa didn't work either.

After a lot of Googling and reading other people's experiences, I came up with a usable configuration for the graphics server. I'm posting here some excerpts for other people, in my situation, to use. Also note that the depth has to be 24 bits, sunffb doesn't work on other color depth.
Config file:

Section "InputDevice"
        Identifier  "Keyboard0"
        Driver      "keyboard"
        Option     "XkbRules" "sun"
        Option     "XkbModel" "type5"
        Option     "XkbLayout" "us"
EndSection

Section "InputDevice"
        Identifier  "Mouse0"
        Driver      "mouse"
        Option     "CorePointer"
        Option     "Protocol" "BusMouse"
        Option     "Device" "/dev/sunmouse"
EndSection

Section "Device"
        Identifier  "Card0"
        Driver      "sunffb"
        Card        "3D Labs"
EndSection