Hardware

Is anybody running m0n0wall on a PCIe CPU board?  Something like this- http://www.totalcontrolsolutions.com/single-board-computer/TCS-001-01533

I want to rent a 2U space in a colocation center for an ESXi server.  I don't want to need any additional space for my firewall, so I thought I could put a CPU card right in the server and install m0n0wall on it... 

Can it be used this way?  I guess all I need from the PCI bus is enough power to run the card.

You can't just plug it in to a pci slot on another computer and expect it to work.  It is designed to be a standalone pc in a back plane configuration.  You could, however use an embedded type board and power it from your 2u power supply and run the interface cables out the open slot. Heres an example:
http://www.totalcontrolsolutions.com/single-board-computer/TCS-001-01698

This should have plenty of power to run a firewall and not put off too much heat.  Especially if you mount it to the lid of your chassis.