as for maya, i would not recommend any newer version for the configurations you mentioned. maybe 2.x? the scripted interface alone is pretty cpu intensive and therefore unresponsive if you're dealing with it for a while. on octane2, it's very useable and suited for daily work - the CPU makes the big difference - maybe if you have an o2 with R12k, it could make sense.
soft3d, well 3.8 SP2 runs pretty good at max impact, they switched the interface-look somewhat in 3.9 and newer versions - never seen that running on an SGI though and so don't know if it makes a difference in performance. bear in mind that soft3d is pretty good at animation and might do a good job at rendering, too - but the whole modeling/dynamics/particles/weighting-stuff is pretty outdated by todays standards.
afaik, you cannot get older releases - they are trying hard to convince you to make the move to XSI anyway