Nowadays processor is more important than RAM. If you have at least 2-3GB of RAM, get processor.
I have 4GB of RAM, but this is my setup right now:
Channel 0: 1GB 667 MHz PNY DDR2 RAM
Channel 1: 2GB 800 MHz Corsair Dominator DDR2 RAM
Channel 2: 1GB 667 MHz PNY DDR2 RAM
Channel 3: none
I have "flex memory mode" which allows for different speeds to work with each other but poorly. Result = Not much performance.