A minimum of 64 MB Random Access Memory (RAM) is recommended. RAM is a very important component of your computer system. You know your system needs additionally RAM when it slows down significantly and the hard disk seems to run constantly.
Be sure to leave room for expansion if you are buying a new system. Ask the vendor how the memory is configured and request your memory in a configuration that leaves slots open for additional memory. If the existing configuration uses all available memory slots, then you'll end up discarding some of your old memory if you want to add more memory in the future.