[Intuition]

I'd get at least a 300mhz r12k O2. I have both 300 and 400 mhz o2. The 400 I have seems significantly faster/snappier than my 300. Though even a single proc 195mhz Octane 1 is snappier than them both as I started with that and upgraded to a dual 400mhz proc for the Octane. Now its very very snappy and is a pleasure to do 3d in considering the era I am using.

I wouldn't want to try any slower than 300mhz r12k o2 proc.

I have been doing 3d Animation for a living since 1995 so I am going to be a bit more picky about machine snappiness than most in this consideration for use old SGI for retro CGI uses.