A brief listing of models and their most important specs:
DECstation 2100 - 12.5 mHz R2000 CPU, a max of 24 megs of RAM, mono or 8-bit color framebuffer, SCSI-I controller, Lance ethernet, four serial ports
DECstation 3100 - Same as the 2100, but with a 16.67 mHz R2000
DECstation 5000/20, /25, or /33 - 20, 25, or 33 mHz R3000 CPU, a max of 40 megs of RAM, integrated 8-bit color framebuffer, NCR SCSI-2 controller, Lance ethernet, two option slots, sound, floppy drive
DECstation 5000/50 - Same as the other 5000s, but with a 50 mHz R4000
DECstation 5000/120, /125, or /133 - Same as the other 5000s, but with three option slots and a max of 128 megs of RAM
DECstation 5000/150 - Same as the 5000/50, but with three option slots and a max of 128 megs of RAM
DECstation 5000/200 - 25 mHz R3000 CPU, a max of 480 megs of RAM, NCR SCSI-2, Lance ethernet, three option slots
DECstation 5000/240 - Same as the 5000/200, but with a 40 mHz R3000 and faster option slots
DECstation 5000/260 - Same as the 5000/260, but with a 60 mHz R4400 CPU
So these are all MIPS-based machines. In addition to the previously mentioned operating systems, you can run NetBSD or Linux on your DECstation.
NetBSD seems the wiser choice out of the free UNIX systems, because it more fully supports the hardware.
This information was summarized from the Linux DECstation page, available at http://www.xs4all.nl/~vhouten/mipsel/models.html.
That page has more specifics, plus information on what Linux supports and how to shoehorn it onto your system. Good luck :)