i'm not expert so i can tell you just my experiences. i'll like that someone with knowledge can arbitring what we talk here?
i instal my new asus p4c800 deluxe and p4/3.0 few days ago.
first i instal xp as "acpi multyprocessors pc" (2cpu's and 22 irq's) and i have problems with sfp instalation. all the time i had error mesagges "can't find dsp file "xxx", please check your instalation.
then i tryed to update computer to "acpi pc" and then i had what i have now (2cpu's and 16irq's). sfp works without any problem, but i had some minor probs with my uad-1 card. than i tryed "standard pc" mode (no cpu's in device managger and 16 irq's). in this mode sfp works nice too, but i wasn't able to instal uad's drivers and shutting down computer properly, also i didn't had virtual cpu's.
than i tryed some "mps uniprocessoes pc" mode. this mode was total crap.
and in the end i instal "acpi uniprocessor pc" (1 cpu and 22 irq's). this mode works good, specialy my uad-1 works better than ever, but sometimes sfp didn't startup properly.
so, i will leave system in this mode for sometime. i will like to try "multiprocessors pc" again if someone teach me how to force system to work smooth.
maybe i will try to instal computer as "acpi uniprocessor pc", then after all drivers, programs instalations and tweakings i will try to enble HT in bios, and update computer to "acpi multiprocessors pc" (if is possible).