You have another (cheaper) option than a router. Simply make the PC closest (say machine 1) to the ADSL connection the gateway machine and use a wireless card (or usb device) in both machine 1 and the other machine (say machine 2).
That way, while machine 1 is switched on, you have a wireless network with machine 2 and then machine 2 can use the internet connection by going through machine 1.
However, the wireless router would be the only option if you wanted access to the internet from any machine without having to have another machine switched on.
On this set up, simply hardwire the router to the nearest computer, buy a wireless network card for the second machine and you're sorted.
You can buy a combined ADSL modem and wireless router from a variety of manufacturers, such as Netgear and Belkin.