Hey ajschmitt,
I downloaded the manual and in there it says;
"If you install four 1 GB memory modules, the system may only recognize less than 3GB because the address space is reserved for other critical functions. This limitation appears on Windows XP/Vista 32-bit operation system which does not support Physical Address Extension (PAE)."
"If you install Windows XP/Vista 32-bit operation system, a total memory of less then 3GB is recommended."
Of course there could be away around this, I am planning on doing a P5K build as well and would like to get 4 gigs in there myself. I will let you know if I find anything else.