thanks to my weird home lan setup i use both of them at once.
it goes like this
internet over lan -> Switch ->my pc
.................................. |.......|
...................................|----------> Router -> Ps3 + 2 notebooks + dad's pc

that way I've got 4gb(day)/8gb(night) internet on my pc(i only get internet via switch - not from the router), access to my home network, and my parents + ps3 got separate 4gb(day)/8gb(night) connection.
and yep - second Ethernet port was really useful on lan-party too.