The graphics card ALWAYS 'hangs off the north chip', since AGP days on. The problems with bus contention aren't due to 'bandwidth' per se, but due to latency overhead with certain drivers and hardware addressing (specifically chipsets like Nforce4 have some serious problems in their hardware). With more stable chipsets this is less of an issue and using Doubledawg to clock down the PCI latency of the graphics card can be unnecessary, but can still yield improvements (masterverb test tells all).
And as for Xp's strange allocation of interrupts, with a modern 2.x APIC controller on your motherboard and Xp's ACPI implementation (as of sp1/2) you should have NO REASON to do 'standard pc' mode. This is something leftover from Win2k days (old ACPI) and older APIC hardware implementations. ACPI/APIC supercedes any pre-ACPI ideas of how irq's work btw, go read up on it at ars techica if you're interested.
Basically, a modern Intel or AMD chipset shouldn't be installed this way (non-ACPI) unless it's an absolute LAST RESORT. There's too much modern technology being bypassed at this point, and provided you choose your hardware somewhat carefully there should be absolutely no reason for it to not work. A bit of shuffling of PCI cards is the usual thing to do, as you have seen. Usually I map out my 'irq sharing' by using a boot cd into some linux or DOS OS mode that is devoid of ACPI. The IRQ's are not the important thing, what is important is to see what cards share where, I find using a stripped down OS boot disk to be the easiest way to do this (or you can even just hit pause/break during the boot POST screen if you're fast enough).
Creamware uses the standard PC answer as in support emails out of habit. It's a fast fix that's really unnecessary but since people often prefer teh quick fix over the proper one it's been repeated here ad naseum.
Anyway, happy to hear that with some system tuning things are looking more stable.
