i think it's mainly a geforce thing (or maybe even a tnt thing also). the video card doesn't like to share the irq and the first pci slot shares with the agp slot. it does seem stupid but i think it's only a problem with agp cards that need their own irq
Take a look at the KT7 FAQ at: <A HREF="http://go.to/kt7faq" target="_new">http://go.to/kt7faq</A>. Click on the <b>Instability</b> link on the left and then scroll to the article called <b>How do I avoid IRQ Conflicts?</b>.
Obviously much of this is KT7-specific, but there's a good, brief discussion on the PCI standard and IRQ sharing.
I wouldn't say that IRQ conflicts are limited to any particular piece of hardware. Most of my add-on devices needed IRQ tweaking before they would work properly.
So, having PCI slots share IRQ is evil -- but considering the limited # of available IRQs in the first place, probably a necessary & unavoidable evil