The 8mb is a cache that it built into the CPU, which is totally different than RAM.
As for RAM though, depending on what version of Windows you are running and whether it is 32 or 64 bit, it might be advantageous to add more. With Windows XP, having 4gb is nice. For Vista or 7, 64 bit, I recommend maxing it out.
You can run Crucial Memory's system scanner to find out what you have and how much you can put in and what speed.
http://www.crucial.com/
If you are running 32-bit Windows, 4gb is the limit. If 64-bit, you can add whatever amount the motherboard can handle.