Our AM5728 motherboard includes two PCIe sockets
so the effort there is lower than on an X15; you just plug compatible expansion cards in the socket.
Plugging aside, it's worth reading the 5 PCIe errata before going too far, and bearing them in mind:
* i847 might catch you out with low drive level depending on which rev chip your X15 has
* i870 might catch you out if registers are byte spaced (eg. a serial port arranged to look like the original IBM PS/2 peripheral)
the other 3 errata are pretty benign,
James.