Aces Hardware has put up an article going over the various multi cpu systems that are available now or in the near future. It covers quite a wide range of platforms from the upcoming AMD Hammer to the tried and true P3. This is a very extensive guide and long read. There latest and final installment is located Volume Multi-Processor Systems: Part 3. If you'd like to start from the begining, you can go here to Part 1 and Part 2.