If you're going for the regular BT ADSL offering it comes in on USB. Cheapest solution would be to have one machine managing this connection, with some firewall software (Black Ice from Internet Security Systems -
http://www.iss.net is a good one). Then connect the 2 PCs via an Ethernet crossover cable - give them IP addresses and download some free proxy server software (try
http://www.analogx.com/) to run on the ADSL connected PC. Point everything from the second PC to this. Works for me and avoids requirement for a separate hub or router.
Good Luck