Yes. you can have multiple network cards in your PC at the same time (wireless via USB, PCI wired). In fact, many notebooks today come with a default of a wired connection (so you can plug in when you are in office and you have a network cable) and a WIFI card (so you can connect if you in range of a WIFI station).
Operating systems like Windows XP make it easy to install these cards quickly. The network configuration also can be done very easily (mostly done automatically for you.
So overall this should be relatively easy for you to do:
1) get the wired LAN card, install, and connect it
2) get the wirless LAN card, connect to USB, install it
3) for the network configuration you can use defaults.
That's it. If one of these get disconnected the operating system should be smart enough to reconfigure for you automatically.
Hoep this helps and answers your question.