To my knowledge, ACPI stands for Advanced Configuration and Power Interface - power management. I believe you are confusing MPS for SMP, or Symmetric Multi Processing, which is the only type that W2K does. When people refer to multi processing, they are almost always refering to SMP on this forum.
As for your second question, I belive that W2K has to be reinstalled, but that there is a way around that. If you have the SMP kernel running and remove a processor, I am not sure of the results.