That's what bridged networking is for. It makes your one ethernet card look like two or more. A single ethernet card can listen to traffic sent to multiple IP or MAC addresses. For example, my MacPro is connected to my router with a single ethernet cable. However, with two virtual machines running, my router sees three computers:
Code:
192.168.0.190 joevt-Ubuntu1 00:1c:42:64:d0:3b
192.168.0.191 JoeWindowsXP 00:1c:42:9c:45:dd
192.168.0.193 JoeMacPro 00:1f:5b:39:c8:bd
Leave the Mac settings alone in the Network system preferences panel, and just setup the network adapter in the virtual machine configuration to point at your Mac's connection.
The network adapter settings in the virtual machine tell which network connection to use and what MAC address the bridged connection will have (which should be different than your Mac's ethernet MAC address). Then you set the IP address in Ubuntu.