It's rather complicated to explain, but it basically is affected by what motherboard you use and how you map out the PCI_IRQ mappings in the BIOS. In almost all motherboards produced today, PCI1 shares IRQ_A with the AGP slot, PCI2 has IRQ_B exclusively, PCI3 shares IRQ_C with the onboard hard disk controller, and PCI4, 5, and 6 if you have it, all share IRQ_D. The actual layout varies from mobo to mobo, so check your manual. I'm pretty sure this is the way it is for all Abit motherboards as well as most Asus.
So, let's say you've got a video card, sound card, and network card. The video card obviously goes in AGP. Skip PCI1, and put the sound card in PCI2. Put the network card in PCI4, and voila. Everything should be isolated. The problems come into play when you start adding more devices to the system like a DVD decoder and such. Now, it's not likely that you'll be using it during heavy network activity, so you should be okay with it sharing an IRQ with the network card.
SCSI causes a problem if it doesn't get its own IRQ, so if you've got SCSI, then your best bet is to put it in PCI2, and move the sound card to share with the network card.
If you've got onboard motherboard resources thrown in the mix, then it's rather likely they'll be taking up IRQs too, like an onboard ethernet controller, for instance. It gets to be a lot to juggle, unfortunately.
Hopefully this has been of some help.

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