Is your setup PPPoE?
I know for a fact that my local DSL provider's modems will assign two IP addresses even though they say they won't, using the same account. So with a simple hub all you have to do is install the standard PPPoE software and it works - doesn't matter if it's Mac or PC. It's no different than using one computer. However, it's a Nortel modem and is limited to two computers, and I know other DSL providers will block this. YMMV.
As for TCP/IP for internet connections on a Mac through a router, it's simply setting it for DHCP and TCP/IP and everything works fine. But a router is $$. Dunno about setting up one computer as a gateway, but I suppose you could always try it and see what happens.
File sharing is a completely different kettle of fish though.