Smurf is right, you actually don't need an IP address on your second NIC. When Wireshark is installed, the WinPcap application installs it's own TCP/IP stack which runs in promiscuous mode. You can then run Wireshark on your second interface with TCP/IP disabled or unchecked in your network properties. I actually just uncheck everything. This is also nice because it would prevent any traffic originating from your second NIC from showing up in your packet captures (DHCP, ARP, NetBIOS, and other chatty protocols).