When deciding on a computer, the processor is the most critical part of a system, as it determines the maximum possible speed that the computer can run at. There are many sockets that CPUs are made for, and many processor models for each socket. The sockets that are currently available are AMD AM3, Intel LGA775 (soon to be phased out), Intel LGA1156, and Intel LGA1366, while the actual models are Athlon II X2, Athlon II X4, Intel Core 2 Duo, Intel Core i5, and Intel Core i7.
So which is the best? What one do you need for your application? As a general rule of thumb, the AMD X2 processors are the cheapest, and still fairly fast for office work, web browsing etc. If you want to play games, work on 3D drawings, or process large amounts of data, go for the Intel Core i5 processor, as these are considerably faster, and even though more expensive than AMD, the best value for money. If you want the top of the range and don't mind spending the dollars, go for Intel Core i7. They are the fastest processors out, but you do pay for the privilege!