If you want to know the "real" physical IRQ sharing then take a look into your mainboard's manual. There should be a table listing the physical IRQ lines and how they are allocated among onboard devices and PCI slots. The latter helps to find a slot that doesn't share too much with other devices.
In this example of my old mainboard you can see that "PCI slot 3" does not share it's IRQ line with any other device. Unfortunately that is only half the truth, because the lowest PCI slot usually shares it's IRQ line with the graphic-card slot (aka highest slot), and so it did on this board.
On the other hand "PCI slot 2" only shares its line with the second LAN controller, which usually isn't needed anyway. So this is the perfect slot for an audio interface once the LAN controller is disabled.
But even using "PCI slot 1" didn't cause any problems with audio interfaces at low latencies on neither Windows XP nor Vista. The resulting IRQ table looked like follows and there were no problems going down to lowest audiobuffer settings on neither the PCI based Audiophile 2496 nor FW based Fireface 400 used in this system.
