Power!!
My current box cost me $500 about three years ago. Fine for most of what I do, but it struggles with 3D. So, with a significant payment for an extra job that I did at work, I decided to upgrade my hardware. My new Dell system is on the way - Core i7 processor, 6GB DDR3 RAM, 64 bit OS (Vista, but I'll survive that), 1TB harddrive etc. Basically, state of the art. I didn't pay an extra $1000 for the extreme CPU, but what I've got should be pretty good for both modelling and rendering. The four cores run eight threads, so rendering should be a blitz.
Meanwhile, I'm working on the eye. There's a free download tutorial (four movies) on modelling a realistic eye from the Gnomon Workshop. I've spent quite a bit there on tutorials in recent times, so it's nice to get a freebie for a change.