Until it died recently I had a Sun Blade 100, circa late 2001 era, 500mhz Sparc IIe with 2 GB of ram, (2) 15 GB HD's, and the stock peripherals, including monitor, which I still use on Intel equipment; It ran flawlessly with both FreeBSD for sparc64, and OpenBSD; I think OpenBSD is the best choice as X works right away from the install.
See this link for downloading:
Index of /pub/OpenBSD/5.7/sparc64
Pick the install57.iso, burn it to a CD and boot that on your system.