NAT with one network card?

[Jeff Hallgren <jhallgren@abaton.com>]

Quick question:
        All of the cookbooks/Howtos I've seen for setting up NAT on a
linux box
assumes there are 2 ethernet cards in the box with different IPs... 
        Isn't it possible to assign an additional (private network) IP
to an
existing card already using the ISPs static IP with ifconfig? Then set
up some addtional routing for it.?
        Or do I really need to install another card?
Thanks!
--
Jeff