I really don't want to spell it out, since it sounds like you are doing a school project. In fact it would be easier to buy a Motherboard POST card if you just need one to use.
The idea is quite simple. The 74hc394 Latches that Address and Data lines, (or stores the data on the bus since the PCI bus is constantly changing). Since you only want the debug value, the 74F521 compares the latched data to port address 80h, (you could put a dip switch to configure to other port numbers), and the Led's give the binary code. If the latched data wasn't port 80, the latached data is not outputed to the led's.
Since you really don't want to convert binary to hex in your head, add a decoder circuit to convert binary to hex for a seven segment led. I didn't design the circuit btw.