What do the different /proc/bus/pci/0x directories represent?
Do they represent the different PCI/PCI-E sockets on a board?
If I have one mini pci and 2 PCI-E sockets on my board, should I be seeing 3 different '0x' directories under "/proc/bus/pci"?
Yes, that maps to your hardware, it exposes the PCI system data. But that interface is deprecated in favor of the /sys interface.
But you really should use the lspci command to find out about your devices, and not read those files directly.
Answer from a friend - They represent PCI/PCI-E bridges present in the system. Usually each PCI slot is connected to a bridge - so it should show up in /proc/bus/pci. But its likely that /proc/bus/pci has more directories than PCI slots you can see