Before we dive off into that, here is a link to some ideas of hacking with old sun gear. By hacking, I mean writing my own bare metal software, learning how the bootrom works, and such things.
Once the cream of the hardware that ran unix, these have quite definitely passed into the realm of antique hardware and retrocomputing. The old sun3 and sun4 models that I used to use and treasure are now quite absurd with 4M of ram and (on the fastest models) 25 Mhz processors. With the advent of fast cheap PC's that run linux, there is little reason to use even the most modern Sun Ultra's (which are 300 Mhz sparc processors with something on the order of 256M of ram. The usual excuse is the need to run some unique third party software, or in-house software that bad management has neglected to migrate to a modern system.
Lastly, let me offer you an old copy of the Sun hardware FAQ that was once maintained by James Birdsall. It is a real treasure for anyone that wants to try to fire up old sun hardware.