RAM usage in blender 2.5?

Hi, I’m thinking about bying a new computer and I’ve got the choice to pick either 12 GB RAM och 18 GB RAM(About 140 USD difference). So my questions is,

  1. for what does Blender use the RAM?

  2. will there be a noticeable difference between 12 and 18 GB?

  1. Hair, multires, fluids, smoke.

  2. The more, the merrier.

  3. A solid state disc (SSD) is recommended for speed. Blender auto-saves all the time and a SSD will eliminate the lag.

i have thought about this, high poly renders take quite long on my comp even though its pretty badass, the part that takes the longes is the “raytree building” stage during which blender slowly fills my RAM.
it fills it sloly and it stops at 50%, it keeps going and going but it never uses more than 50% of my ram, is there a way to increase this and make it faster?