Are you trying to upgrade a system, or building a new one? If you are upgrading a system, you are limited to whatever fits in the socket that you have on your motherboard. If you are building a new one, you can go with anything, though I'd personally stick to an i series part or and amd phenom II or athlon II. That choice depends on what you are going to be using the computer for, so if you are building new please tell us what your most demanding application will be.