Hi Nate,
I know this is sort of a loaded question, but I don't have a lot of knowledge on distros that are stable on Sparc.
I *think* debian is the only "major" distro left that supports SPARC to the fullest extent. Though watch out if your running the latest T1/T2 type processors, last I checked kernel support for those was somewhat limited still.
It has two 1.5 GHz UltraSPARC processors, from what I see.
-Jason