Like Noriaki stated, I have always seen the AGP slot, PCI Slot 1 and 5 all share the same IRQ. I changing the IRQ assignments for Slot 1/5 results in the same IRQ for the AGP slot, at least with BX/Award BIOSes.
Supposedly it would work, but my experience says that especially with 3D games, the sharing of the PCI sound card or network adapter with the graphics board is not something you want to do (I have an Nvidia board) - it results in intermittent pauses in the graphics rendering. 56K PCI modems however, seem to share well with the video card.