Yes - you either need 2x network cards or 2x IP addresses on one network card. I would go for the latter.
However, to do this you do need at least 1 public static IP address.
Have your internal IP address (192.168.0.10 for example) as your primary address and the public IP address as the secondary.
Now, I am presuming that you are using a router with a firewall or something?
Yor router will need 2x IP addresses - one on your network (192.168.0.1 for example) and another on the internet - that IP address must also be static.
In the IP settings on your workstation set up two gateways. Gateway # 1 (which will be used by your primary IP address) needs to be the internal IP address of your router and have metric of 1. Gateway # 2 (which will be used by your public ip address) should be the public ip address of your router, and have a metric of say 10.
So far so good....all outgoing traffic will go via your primary IP address (192.168.xxx) and appear on the internet as being the public ip address of your router.
Incoming traffic, if allowed through by your firewall will obviously go to the public address.
M.
Hollingside Technologies, Making Technology work for you.