I wouldn't bother. I've a Sparcstation 20 sitting upstairs in my loft, unused. Why? It's noisy (fans, narrow SCSI hard drives), hot, and slow. The neatest part of it is the BIOS. Additionally support for sparc32 is being dropped from various Unixes (OpenBSD has just binned it). I used it as a firewall for a while, before its 10Mb network card became an issue (quad port 100Mb is expensive, as Sparc32 doesn't take PCI).
Sparc64 isn't much better, but has the advantage of being a tad more modern, supported by various OS (check the graphics card support though) and useful sometimes for checking endian issues.
Again though, Sparc64 is usually hot, slow, uses fibre channel disks in some Ultrasparcs, and has gone through the cheap phase (at least on ebay UK) as is climbing back up again. I regularly ponder getting one, each time the reluctant conclusion is no. If you see a particularly cheap Sun Blade 150 it may be worth it - it's very slow, but quiet, and supports IDE disks.
The only non Intel systems worth getting these days are various ARM devices, or a late model PowerMac (still slow, but not unusably so, depending on what you're doing). There are also development POWER systems if you have comparatively deep pockets.
The other option is SGI as mentioned. If you can get either a cheap O2 with a reasonable amount of memory, or a cheap Fuel, it's probably worth a play. Octanes are very noisy indeed, and have a lot of different options, most of them quite limiting in terms of the software they'll run. Disks are SCSI again.. I got a rather good bargain : two O2s for a tenner each(!), both with the full AV cards, and about 640MB memory across both systems. The disks are horribly noisy, as is the power supply fan until you mod it.