Thanks to all those that replied. The winning answer is as follows:
Framebuffer Resolution Guide
----------------------------
What follows is a compilation from other sources of what resolutions
the Sun Framebuffers support, and how to put the framebuffers in the
resolutions.
Resolution ZX GT GS GX GX+ TGX TGX+ CG14 A24
---------------+-----+-----+-----+-----+-----+-----+-----+-----+-----+
1600x1280@66 | | | | | | | yes | yes | |
---------------+-----+-----+-----+-----+-----+-----+-----+-----+-----+
1280x1024@76 | yes | yes | | | | | yes | | |
---------------+-----+-----+-----+-----+-----+-----+-----+-----+-----+
1280x1024@67 | yes | yes | | | yes | | yes | | |
---------------+-----+-----+-----+-----+-----+-----+-----+-----+-----+
1280x1024@66 | | | | | | | | yes | |
---------------+-----+-----+-----+-----+-----+-----+-----+-----+-----+
1152x900@76 | yes | | yes | yes | yes | yes | yes | yes | yes |
---------------+-----+-----+-----+-----+-----+-----+-----+-----+-----+
1152x900@66 | yes | | | yes | yes | yes | yes | yes | yes |
---------------+-----+-----+-----+-----+-----+-----+-----+-----+-----+
1024x800@84 | | | | | | | | yes | |
---------------+-----+-----+-----+-----+-----+-----+-----+-----+-----+
1024x768@77 | | | | | | yes | yes | | |
---------------+-----+-----+-----+-----+-----+-----+-----+-----+-----+
1024x768@76 | yes | | | | | | | | |
---------------+-----+-----+-----+-----+-----+-----+-----+-----+-----+
1024x768@70 | | | | | | | | yes | yes |
---------------+-----+-----+-----+-----+-----+-----+-----+-----+-----+
1024x768@66 | | | | | | | | yes | |
---------------+-----+-----+-----+-----+-----+-----+-----+-----+-----+
1024x768@60 | yes | | | | | yes | yes | yes | |
---------------+-----+-----+-----+-----+-----+-----+-----+-----+-----+
Stereo
---------------+-----+-----+-----+-----+-----+-----+-----+-----+-----+
960x680@108 | yes | | | | | | | | |
---------------+-----+-----+-----+-----+-----+-----+-----+-----+-----+
960x680@112 | yes | | | | | | | | |
---------------+-----+-----+-----+-----+-----+-----+-----+-----+-----+
960x680@120 | | yes | | | | | | | |
---------------+-----+-----+-----+-----+-----+-----+-----+-----+-----+
PAL
---------------+-----+-----+-----+-----+-----+-----+-----+-----+-----+
770x575@50 | yes | | | | | | | | |
---------------+-----+-----+-----+-----+-----+-----+-----+-----+-----+
NTSC
---------------+-----+-----+-----+-----+-----+-----+-----+-----+-----+
640x480@59.94 | yes | yes | | | | | | | |
---------------+-----+-----+-----+-----+-----+-----+-----+-----+-----+
A Note about the 13W3 connector and Sun Framebuffers
----------------------------------------------------
Besides the red, green, blue and sync connections, the Sun monitor
connector (called 13W3) has several sense pins which allow the
monitor to tell the framebuffer what resolutions and frequencies
it can handle. The sense pins are pins 3 (S2), 8 (S1) and 9 (S0).
These are used in combination with pin 4 (GND) to communicate the
resolutions to the framebuffer. The table below is a list of how
the pins are interpreted. Many of the framebuffers will not allow
user to put them in a mode that the monitor says it does not
support. If trying to put the framebuffer in a particular
resolution but it won't go, check to see if the monitor supports
the resolution.
Scan Rate Code S2 S1 S0
---------------+-----+-----+-----+-----+
1152x900 66hz | 7 | | | |
---------------+-----+-----+-----+-----+
1152x900 76hz | 6 | | | GND |
---------------+-----+-----+-----+-----+
1024x768 60hz | 5 | | GND | |
---------------+-----+-----+-----+-----+
1152x900 76hz | 4 | | GND | GND |
---------------+-----+-----+-----+-----+
1152x900 66hz | 3 | GND | | |
---------------+-----+-----+-----+-----+
1280x1024 76hz | 2 | GND | | GND |
---------------+-----+-----+-----+-----+
1600x1280 76hz | 1 | GND | GND | |
---------------+-----+-----+-----+-----+
1024x768 77hz | 0 | GND | GND | GND |
---------------+-----+-----+-----+-----+