Even so, a couple of Gigabit network cards, a crossover cable, and some statically assigned private IPs would be better. This new "network" is completely isolated from everything else just like it would be if you were using USB, only a lot faster and more "standard" as there is no extra software installed.
The end result is identical, you are just using an ethernet cable instead of a USB cable. I have a client site set up like this. Their regular network is a non-switched 10/100 network, but the web server talks to the database server over a dedicated GigE connection via a crossover cable. The regular network is a 192.168.0.xxx address range and the two GigE cards have 10.1.1.xxx addresses. I adjusted the hosts file so they will always chatter to each other over the private connection rather than the regular network.
It runs transparently and is isolated from the rest of the network.