"peripheral card" SBC with Linux driver support?

I would love to have an independent computer inside a regular computer
to perform various monitoring and control tasks.  The "peripheral card"
in this article sounds like what I want.
    http://www.embeddedintel.com/display.php?article36 %

Ideally I'd like an SBC that I can stick in a standard PCI slot.  It
would boot Linux from an onboard CompactFlash and have a couple Ethernet
ports.  I know those exist.  The tricky part is that I want to be able
to make the SBC look like a standard serial adapter to the host
computer.  It would be *great* if it could also look like a display
adapter.  And if it could appear as a network adapter I could eliminate
one of the Ethernet ports (which would have just been connected to the
host computer's network adapter).

Does something like that exist?  I'm not finding anything on SBCs
emulating other PCI devices but it seems like it should be a fairly easy
thing to do.

Thank you.

--kyler