Specs are :
QED R5200 300Mhz CPU (upgradeable to R7000 600Mhz with some geeking :-)
128Mb RAM (probably more actually .. won't be less though and expandable to 1Gb)
36GB 15,000rpm SCSI disk (can take two drives)
IRIX 6.5.29 (Or can run Linux, OpenBSD or NetBSD)
CRM graphics with up to 900Mb or so of texture RAM (if you have 1GB main memory)
VICE (Video Image Compression Engine) does hardware compression/decompression for video and images.
64bit PCI slot of sounds cards, firewire, ethernet, SCSI Firebchannel etc.
Audio module installed
Makes a very nice workstation, and I'll load it up with Firefox, OpenOffice, GIMP, Blender and a bunch of other free stuff.
Make me an offer, it's currently living in Auckland and a pickup would be best.
For more info on the machine, look on Wikipedia : http://en.wikipedia.org/wiki/SGI_O2