Replacing the processor with a faster model is one of the most effective and cost-efficient upgrades you can make on an older system. In some cases, you can double or triple CPU performance at a relatively small cost. Unfortunately, not all systems are good candidates for a processor upgrade. You'll have to do a bit of research to determine whether your system is suitable for a processor upgrade. Here are the factors to consider:
The first consideration is the socket type provided by the motherboard. Motherboards that use a current socket—Socket 775 for Intel or Socket 939 for AMD—are the best upgrade candidates. Motherboards that use older sockets—Sockets 462 (A) or 754 for AMD or Socket 478 for Intel—offer fewer processor choices, but are still reasonable upgrade candidates. Motherboards that use very old sockets, such as Intel Socket 370, are poor upgrade candidates, because few processors are still available for them. Motherboards that use obsolete sockets—Socket 7 and earlier, Slot A, or Slot 1—are not realistically upgradable. Even if you can find the components you need to upgrade these obsolete systems, the price will be high, and even after the upgrade, the system will be too slow to be useful.