PCI-Express Processor/Co-processor cards
PCI express is really too slow for this. You need a motherboard designed to take multiple CPUs. These are available for up to 4 CPUs, although this is not cheap.

I believe the card you linked to is a "single board computer", so I'm not even sure it would work in a normal PC. It's probably designed for connection to a backplane or "mezzanine" system.

You can get "exotic" processors and FPGA add-in cards, e.g.:
http://www.nallatech.com/PCI-Express-FPGA-Cards/pcie-287n-7-series-fpga-network-processing-card.html