if youre considering sgi's at all, you need to factor in that with most of the used ones on ebay, you don't get IRIX media, which is very expensive to purchase through sgi themselves....
for your purposes, sgi's are rather good with 3d and visualisation tasks, but it depends on the age of the machine and how intensive the work is youre going to be doing, if you want affordable but not really 'ancient' sgi machines, you miht want to check out used Octane systems froma reseller, they can be had rather cheap if you know where to look

theres a plcae in Australia where i got my used Octane from to learn about sgi's, called network remarketing, they deal in US $ and will ship to the US as well, www.networkr.com.au is there website address, they have octanes with KB and mouse for a reasonable price, im sure you would be able to cluster 3 or 4 of them and distribute your rendering across multiple machines, like what they do at Pixar and special effects plces like that....

i dont know much about sgi's yet, not much about rendering software (if anything), but its just a suggestion you might want to consider