Depends what you want to spend, but my personal choice is a router.
This will sit between your 2 PCs and will take the modem connection and the connections from each PC. It will automatically sort out "routing" of data from the modem ( both PCs can connect to the net at the same time) and the other PC. It will also act as a firewall to your "home LAN".
It also means that you don't have to have one PC as a server (i.e. always on), the router acts as the server so each PC is independant of the other.
However the knowledgeable folk here may have a more suitable solution for you.