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Firstly the Linksys wireless routers, such as the WRT54G allow both wireless connectivity, and also have 4 LAN ethernet ports so you can hard wire computers too. A single ethernet WAN port will connect to your modem.
You won't know if your desktop's ethernet card is working until you try to plug a cable into it (cable must have a network device at the other end too
). However older network cards will probably have drivers that can easily be downloaded from the internet. Alternatively a new network card is a cheap (say £10) component, easily added to your PC.
Setting up a new network is a lot easier if you have at least one PC that can be wired connection via ethernet, so you can configure router settings. Once the wired aspects for security etc are set, then wireless setup can follow.
USB1 will allow connections to operate, just a little bit slow. You could order cable and specify that you need the ability to connect by either USB or ethernet, starting with USB but that you will later be switching, as you replace or upgrade your PCs. Many will say that ethernet is more reliable than USB simply as dedicated hardware is running the network, rather than using general computer resources needed for USB.