I'd be tempted to leave the Cobart Qube alone. This would be just the kind of device that might have some strange scsi driver or some onboard something that won't be supported.
Having said all this, I really know little about the Qube, although it looked pretty good for a small office when I read a review a couple of years ago. If folks are routinely installing other operating systems on these things, the bugs have all been found.
Has Sun stopped distributing patches/updates? If so, I guess you don't have much choice.