XP Home only supports 1 physical processor
XP Pro only supports 2 physical processors
However they both regonise a Hyper Threaded Pentium 4 or Xeon as 1 physical and 1 logical cpu, so you CAN use a HT P4 with XP Home (and the OS will show two processors in Task Manager) and you can use 2 HT P4's with XP Pro (OS sees 4 cpu's in Task Manager)