The Athlon XP and Athlon MP are actually identical now. The MPs are just certified to work in multiprocessor mode.
The XP has a few improvements over the old Thunderbird: Hardware Prefetch logic, increased TLB buffer size, thermal diode, full SSE support, and reduced power consumption. In short it's a little bit faster than the old Thunderbird Athlon at the same clock speed, and has reduced power consumption.