Memory (not to be confused with Hard Drive Space) can greatly increase the performance of a computer. Each version of Window has greater demands on memory. My personal recommendation to get decent performance out of a machine is as follows:
Windows 2000 - 512MB minimum
Windows XP - 1GB minimum
Windows Vista - 2GB minimum
This is for a typical end user who uses Word Processing, Email, Internet etc. Intensive applications such as video or photo editing may require more for optimal performance.
There are many components when considering an upgrade for RAM: Type, Capacity, Speed, Pairs, etc. And it can be confusing to find what you need.
There are many types of memory, DDR and SODIMM being among the most common. It is vital when purchasing memory to find the right type, as other types will not physically fit or work in the system.
Each memory slot has a maximum capacity. This can be found either in the manual for the motherboard or from a system scanner
A Great tool to find all the information you need for your PC is the Crucial System Scanner at www.crucial.com
This will tell you exactly what your computer has and has good pricing for the memory you need!