More memory reduces disk swapping and allows larger and more applications running concurrently. Also very helpful when working with large scanned images.
Buy more memory than what you need; memory is cheap. But, don't go overboard. Memory prices drop pretty quickly.
There are limitations to how much memory you can upgrade. This depends upon your motherboard. You will need to determine the type of memory that your motherboard allows. Types of memory include: parity, non-parity, EDO, SDRAM and PC100 SDRAM