The processor (cpu) is the driver of the computer. Processors are usually differentiated by speed, measured in gigahertz. The higher the Gigahertz, the faster the computer will run. You should buy the fastest processor you can afford, but a 3 Gigahertz processor should normally be enough.
The Memory is used by the processor to run programs. The more random access memory (RAM) you have, the better your computer will run when using several programs at once. Your computer should have enough memory to make the most of the processor speed. For a 3 Gigahertz processor, you should have at least 2-3 gigabytes (GB) of RAM.