The processor that you choose will determine, to a great degree, how fast your computer ends up being and how long it remains current. The newest models from Intel and AMD are all available here. Combined with good cooling, high-end processors such as the Intel i7 series and the AMD FX series can provide incredible performance.
Make sure to check the socket on the processor to ensure compatibility with your motherboard. Very fast processors also benefit from advanced cooling, such as liquid cooling.