The biggest difference that I am aware of is the Xeon has built in support for two processors and the Xeon-MP for four processors. From my breif visit to the Intel web site this morning, using the standard Intel chipsets it looks like the P4 is limited to 4GB of RAM and the Xeon is limited to 16GB of RAM. Via other magic, both can use up to 64GB of RAM. The Xeon chipsets also support multiple PCI busses (32 and 64 bit). There are other differences. The Xeon is not something you are going to see in a laptop.