Yes it can be possible to design a PCI card onto which you would put an 8051 type PC. However in order to make the interface to the bus you will need a PCI bridge chip and a dual ported memory chip. The interface would follow this:

PCI <--> BRIDGE <--> DUAL PORT <--> 8051

To make a cost effective board however your 8051 section and direct 8051 support circuitry will have to be worth siginificant value. Otherwise the bridge chip and the dual port RAM will dominate the cost.

Michael Karas