I'd suggest taking a close look at The Lurker's Guide to Video for SGI machines.
Regarding your thoughts of capturing an <uncompressed?> video stream using a single hard drive on the O2's 40mb/s SCSI bus, I'd probably start with the section on How Big is Video?
, followed by Hardware/System Setup for Uncompressed Video I/O: O2 Systems with MVP
Most SGI video options weren't meant to be a stand-alone acquisition that would enable a base hardware platform to be used in video production.
SGI based video capture/editing/output was never intended to be trivial where either money or knowledge is concerned. It was a given their target audience would have the resources to purchase all the necessary auxiliary hardware and broadcast quality equipment needed, *and* hire or train a dedicated user/admin/support staff <they were aiming more for hollywood than home>.
Even at the lower end, SGI hardware isn't particularly consumer device friendly. The O2 is often described as "entry-level". This forum is full of posts describing problems capturing video with the O2, even when fairly high-end consumer video equipment was used. The O2 expects you'll provide sufficient storage capacity/bandwidth and video equipment with features that aren't always typical for consumer/prosumer hardware <such as time based correction>