Ahcix86 3.3.1540.29, as suggested by blackwingcat, can break PAE compatibility in W2K extended kernel (on my system - M5AF99FX PRO R2.0 it does). It seems this driver was written without care of
https://msdn.microsoft.com/en-us/library/windows/hardware/dn613975(v=vs.85).aspx
Perhaps it would be better not to use too many XPSP3 files with w2k exkernel, they seem to be buggy with "real" PAE in native environment ...
Workaround: if you experience i/o problems (I had strange read/write to/from disc problems) with 3.3.1540.29 and /PAE mode, simply replace ahcix86.sys with v3.2.1540.75 WHQL Server2003. It will resolve this (also AMD Raid console will be installed). The system seems to be faster, too, without /PAE.
Now I can use 32GB in W2k Exkernel!!!
Greetings from Austria!