Go into System Information, then click Hardware Resources to expand it, then IRQs, and it'll tell you what everything is using.
Originally, I had a cardbus wifi card, which kept sending XP loopy in addition to freaking out the soundcard. I can't change IRQ slots in my bios so I was stuck with it, bought a USB wifi and it, typically, went straight onto the one the other card was using.
Looking at them now, my Ethernet card uses IRQ11 and I can't ever remember iTunes acting nutty when I was wired.