HD activity can be reduced by monitoring your RAM usage and messing with the bucket size.

with regards to render time, the big hits are reflections/refractions, soft shadows, min shading rate, etc.  depending on what you are doing, you can lower reflection quality to decimal values and still achieve decent results in a fraction of the time.

there is a python script included with Poser that will time your renders so you can render, adjust settings, rerender and see what an impact the changes had both on visual quality and rendertime.

Scripts - Partners - Dimension3D - Render Firefly - Render (Time)

 

 

as for RAM being 4x as expensive in Australia - damn, that sucks. i complain about the 10-20% difference in prices here in Canada but you guys really get the shaft.

...but theres always ebay. heres a 16gb kit for ~$125 shipped to Australia.  keep in mind that that is a result of a 1 minute ebay search, im not vouching for the seller or anything.