Several methods of sharing a DSL connection are outlined at http://sympaticousers.org/menu/nav.htm
All assume an external DSL modem. Note that some DSL modems (notably the Nortel and Alcatel) support more than one MAC address, so if you get a hub, plug in the two computers and the modem, both machines can use the modem. In the latter case, if your ISP uses straight DHCP as opposed to a PPPoE connection (as, unfortunately, more and more ISP's are using for DSL connections), then you can just set up your linux box to connect to a DHCP server, and off you go. If you need to use PPPoE, there are several solutions out there, the easiest is probably from Roaring Penguin, http://www.roaringpenguin.com/pppoe/. The Linux 2.4 kernel has direct support for PPPoE.
