I've recently modelled car wheels and, although I'm pleased with the outcome, they're fairly high-poly and its slowing Maya down and I'm having to manually control the memory allocated to mental ray to avoid errors.

I've read that instanced geometry is less intensive than straight duplication... is this the case? And is it as simple as duplicate special/instance/ or are there other requirements.

Also, although I've discovered how to limit memory allocation during the render (render options>auto memory limit) and have 5gb ram, only around 1 is deemed available when I press calculate, going down to about 500mb after a render. Is there any way I can purge this easily. I've looked it up, but it's all mel commands and similar and I really don't want to divert my focus of getting this first model finished on time to start learning scripting. Is there a simple 'purge' button somewhere?!