XFree, usually, runs fine on theses systems. The table below helps you choose the XFree86 4.2.0 server that matches your graphic adapter:
Graphic Adapter Xserver Sun Creator 3D sunffb Sun Elite 3D sunffb Sun GX suncg6 Sun Leo (ZX) sunleo Sun TCX suntcx Sun Turbo GX suncg6 Sun bw2 sunbw2 Sun cg14 suncg14 Sun cg3 suncg3
There is a catch with the 13W3 video connector that equips some of the framebuffers you may encounter. One may try to plug it on a VGA monitor using an 13W3 to VGA adaptor that is sold on the market. The problem is that SGI and Sun Microsystems do not have the same definition of this standard and depending of the adapter you buy, this may work or not (you know Murphy ...). As a rule, idealy you should try it before you buy.