Have a look through the HCL
and see what USB wireless cards others have had luck with.
As for using a second router as a wireless adapter, yes it is possible. I'm assuming you already have a router in place, in which case you would plug the wireless router into one of the LAN ports on your original router. In general, it will probably work better if you turn off the wireless routers routing functions and let the original router handle the DHCP fucntions.