Couldn't you do this with say, the MSI K8nNeo2 Plainum? It has two different rj-45 ports, or, two different MAC addresses. In essence, windows XP would name these Local Area Connection 1 and 2. So just disable the one you choose not to use at the time, and surf with the other one.
Thats what ive done with one of mine. I drag it back and forth between home and the hospital. Hospital uses proxies and manual IPs, so i just stuck another nic in there and assigned it all the info. Then when i get home, i disable that one and enable the other one.
In the beginning, I had just the 1 connection set to automaticly obtain the info, then if it couldnt find anything, it would assing the hospital info. This worked as well, but took to long to get a connection at the hospital. Computer would site there for a minute or 2 searching for an IP. Ive found that disabling/enablic 2 diff nics was much quicker.