When I got my new box (over twice as fast), the level of complexity within my images increased without me noticing. I, too, was concerned that the rendering time was not considerably faster until I took one of my scenes back to the old box to render and it took days instead of hours. This may be a problem. Another thing to check is for Hyper-threading. This is a new Intel technology that simulates a dual processor on a single chip. I have done experiments on a Dual processor machine (theres a thread on this about 3 weeks ago) but I still don't know the impact on a single processor machine. Go into BIOS and turn it off and re-render (check the time differences)