More RAM can make your rendering 0-10 times faster - depending on whether or not RAM is your current limit. If you have enough RAM to hold the whole scene in, including all textures, adding more RAM will help nothing - what should your computer put in that extra space? But, if your scene doesn't fit entirely in computer's RAM, it will constantly need to read and write parts of it to the hard drive, and during that time the rendering functions will have to wait. Adding RAM will then make that HD activity less frequent or completely disappear, thus speeding up rendering.