The first decision you need to make is how you are going to connect to the internet and there are two options.
1) You continue to connect with your existing PC and USB modem and set up "Internet Connection Sharing" to allow the second PC ('client') to share the 'host' PC's connection. The 'client’ PC can only access the internet if the 'host' PC is running and online.
2) You buy a "Router" with built-in ADSL modem and dispense with your existing USB modem. This will allow either PC to be online independently from the other. With ADSL, you normally require either a router with built-in ADSL modem or a stand-alone modem that can connect by ethernet, but not USB.
This router is an exception and does support a USB modem click here
In terms of wireless hardware:
With option (1) you can either install a “Wireless Network Adapter” in each PC and set up an Ad Hoc wireless network, or connect a “Wireless Access Point” to an existing network adapter in the ‘host’ PC and a “Wireless Network Adapter” in the ‘client’ PC and set up an Infrastructure wireless network. These are the two basic types of wireless network and the “Wireless Network Adapters” are configured to seek available networks of the appropriate type – Ad Hoc or Infrastructure.
With option (2) you would use a Wireless Broadband router bearing in mind the modem requirements (usually built-in or connected by Ethernet, not USB). The PCs then connect to the router either wirelessly with a “Wireless Network Adapter” or the PC nearer to the router could connect with a normal Ethernet cable and only the remote computer connects wirelessly. It is the router which actually connects to the ADSL service, not either of the PCs.