If you accept the relatively slow speed, use it for fun, and got the time -- sure, I wish you all the fun. The case is at least sexy :D. Just don't expect too much from it. Don't expect to do video processing on it esp if you don't run IRIX. If it runs IRIX and you lost the password there are many ways to crack it. I had this problem too with an Octane of mine. What I did IIRC was booting up IRIX via CD, grabbing the passwd file, and cracking it using Crack or Jack the Ripper or sth. Took a few hours on a slow AMD K7 running Linux. Ofcourse, YMMV depending on the password. Or I am confused with a Sun, and I just reinstalled IRIX. Don't know anymore. My Indy's and Indigo2s were all passwordless when I got 'em.
You say you can make it look like Ubuntu. Just forget it. Drop that attitude. If you want X, you are going to run one of the WMs or at best XFce. Making it look like Ubuntu requires GNOME. If you run IRIX, you'll either have an archaic GNOME version from Freeware.SGI.com, a 3rd party one (does exist for at least GNOME 2.6), or your self-compiled one (good luck w/the latter). If you run Linux/MIPS on it, sure, you can get GNOME. I guarantee you it will be absolutely unusable on your machine. I ran GNOME 2.x on an Octane running IRIX. For about 5 minutes or so. Then it was finally started, I looked around, figured it was far and far too slow, and switched back to good ol' 4DWM (which isn't _that_ bad). The machine, with 4DWM ran Firefox fine though as the standard Netscape 4 has.. hmm.. issues. Keep in mind that machine was faster than yours (dual R10000 195 MHz, 384 MB RAM, almost an Octane2, faster than any O2 in existance). From what I read at Nekochan.net (a SGI community) people with O2s are absolutely frustrated w/the slowness of Firefox on O2 @ IRIX. The thing is simply too slow. Some bought new non-SGI CPUs for it (RMD7200 or sth like that) which are a bit faster running at sth like 400 MHz. Also, that is all with IRIX, not with Linux (IRIX will run better on it). Imagine it'd run GNOME then which compared to 4DWM is bloated beast not even worth mentioning. I say, forget GNOME, and give IRIX a shot. IRIX is a learning experience as well, and you can get the full experience from your SGI as it was meant to be. I also gave my Octane away for 100 EUR.. which I've not received till this day heh.
For compiling, you could use CCache (from Samba.org) on it.
Sure, a NeXT is nice. Guess what though... what OS do people run on it? ;)
What I don't like about SGIs is their power consumption. This thing had something like 700W power supply. If that'd be less it could surely function as e.g. a sexy terminal.
(If you need IRIX 6.5 base or 6.5.22, check DC++/eMule or send me PM. Newer than .22 won't run on it.)