Since everything I can find says a MP is identical to an XP architechurally,can it be used in a KT400 MB?

Yes, it should work without problems.
Basically, an Athlon MP is just an Athlon XP that has been tested/validated for use in a dual-CPU configuration.
Also, unlike their AXP-counterparts ALL Athlon MPs have a 133MHz FSB as well as a 60W max. TDP(Thermal Design Power - i.e. max power dissipation).

-------------------------
The opinions expressed above do not represent those of Advanced Micro Devices or any of their affiliates.

---

<center><font color=red>MODERATOR</center></font>

---

AMD Athlon 64 X2 6000+ 89W :: Asus M2N32SLI Deluxe :: 2x1024MB Muskin XP2-6400 DDR2-800 :: BeQuiet! Darkpower Pro 530W :: 2x WD Raptor 74GB/10k rpm, RAID 0 :: ATI Radeon X1950XTX