The rest of your memory isn't usable because you are most likely running a 32-bit operating system. Your 6GB should probably be upgraded, but I would upgrade your CPU first.
Try this one: Newegg.com - AMD Phenom II X4 955 Black Edition Deneb 3.2GHz 4 x 512KB L2 Cache 6MB L3 Cache Socket AM3 125W Quad-Core Processor HDZ955FBGMBOX
And maybe if you can, buy a new mobo.