Hi madcoo!
First question is: does your render stop while rendering, telling you about a "memory allocation failure"?
If not, you don't use more than 600/700 MB of RAM because you simply don't need that.
1. More available RAM does not speed up your rendering process.
2. The only thing that speeds up the rendering process is CPU's power, and obviously the number of CPUs available (a quad-core will render faster that a dual-core)
3. If you want to see a higher use of RAM, you'll basically need to MAKE THE OUTPUT RESOLUTION BIGGER, or USE MANY LIGHT LAYERS (layerblending feature, which is an advanced technique to separately control the emitters during the rendering), or FILL UP YOUR SCENE WITH STUFF so there are many more bounces of light and the calculations are heavier.
If you're not using all your RAM, it only means that there's no need for you to use that.
Well, I say to you: lucky boy. My problem is often that I lack memory, and I run with 4GB too. This because my scene is TOO FULL OF OBJECTS, or MY RENDER RESOLUTION IS TOO BIG or I TRY TO RENDER WITH TOO MANY LIGHT LAYERS.
I hope I was clear enough.
