[Jacques]

For a starter SGI where the focus is SD video I'd go for an O2. Octanes are plenty fast and certainly faster on a like for like CPU spec basis. 

The modded O2 performs well, I did my own mod with a R10k 250mhz CPU as a base and a spare single proc R12k 300mhz pimm I had. 

For 3D work an Octane will be much faster, the O2 struggles a bit with Maya, though does well with Softimage and Lightwave. A 400mhz CPU is best as it has 2mb cache, which helps loads. It's also very expensive!

I reverted my modded 300mhz CPU back to the 250mhz one as it ran slightly cooler and I couldn't really tell the difference...!

R5k CPU machines, even 300mhz ones, are quite bit slower than their R10k/R12k equivalents.

512mb ram is the minimum I'd go for, I have 384mb and it struggles with Maya a bit.