There is no such thing as a CPU mounted on a PCI card. You may be thinking of the old slot mounted CPU's where the processors was on a card that fit into a slot on the motherboard as opposed to the current socket mounting method. Slot mounted processors haven't been made in years as processors now require more contacts than can be reasonably put on a card for a slot interface.
If you want two CPU's to be recognized by windows you will need to buy a new motherboard and a dual core processor. This will be recognized as two logical CPU's by windows.
I remember those back in the day, but that was pre-pentium.
also Risk based systems had a similar design.
but the PCI and even PCIX and PCI-e are not fast enough in the BUS to be able to handle the speeds of the pentiums and above.
i do believe that they may be producing a similar design for servers in the future for CPU expansions of multi CPU, but this is not available just yet.
Actually there were motherboards with two separate cpus powered by two psu's. The psu's were mounted directly onto the motherboard. They were designed specifically for servers and used in corporate networks. At the time dual core and quad core cpu's did not exist and they certainly weren't intended for home use.
We just had a thread a few days ago over in the psu section of the forum asking about dual psu's for a similar system.