simple answer
You don't need a wireless card for your desktop
You connect it using a cable, the only benefit of wireless is portability and you hardly likely to want to walk about the house with your desktop.
Wireless is less secure, less reliable and far slower than wired.
When you get your wireless router simply plug the cat5 cable into the back of the desktop and the other end into the router and it will work without any setup needing to be done on the computer
if you want to use your USB modem until you get the router then you simply need to download the drivers for it, install them and then setup a connection using your broadband username and password. (or if you have the disk that came wth the modem that may work, though the new PC will probably be Vista so it might not!)
You are somewhat over complicating things, you have no need to change anything about the TCP/IP protocols at all, the network setup is essentially the same regardless if used on a home or a business network.