In order to do their thing, the PCI devices in your system need to take advantage of various specific hardware resources in your system, such as interrupts. Many PCI devices take advantage of hardware interrupts to signal the processor when they have some data ready for processing. To see what interrupts are being used by your various hardware devices, you can view the /proc/interrupts file by typing cat /proc/interrupts. You'll see output that looks something like this:

           CPU0       
  0:    3493317          XT-PIC  timer
  1:      86405          XT-PIC  keyboard
  2:          0          XT-PIC  cascade
  5:          0          XT-PIC  eth0
  8:          2          XT-PIC  rtc
  9:      62653          XT-PIC  usb-uhci, usb-uhci, eth1
 10:    1550399          XT-PIC  Audigy
 12:     413422          XT-PIC  PS/2 Mouse
 14:      85418          XT-PIC  ide0
 15:          4          XT-PIC  ide1
NMI:          0 
ERR:          0

The first column lists an IRQ number; the second column displays how many interrupts have been processed by the kernel for this particular IRQ; and the last column identifies the "short name" of the hardware device(s) associated with the IRQ. As you can see, multiple devices are capable of sharing the same IRQ if necessary.

You can also view the IO ports that your hardware devices are using by typing cat /proc/ioports.