Not sure if it'll help you but I frequently do the same as you--indeed I switch between the internal card, a simple 2 channel M Audio USB card and a large mixer feeding via Firewire.
All I need to do is unplug the one I no longer want and it reverts to the internal card. If I plug in the USB box again, the computer detects it and, within about ten seconds, it switches to the USB. The Firewire is more complicated as I'm not supposed to hot plug that--I shout down the computer, plug in the FW and it's detected on a reboot.
The only thing I've done set-up wise is to tell the Windows Audio control to use the USB as the default device when I first connected it and the same with the FW when that was first connected. It seems to remember these preferences and only default to the internal stuff if it doesn't detect external.
Maybe I'm just lucky...hope it's as easy for you.