Onboard LAN versus network cards?

The speed of your network is only as fast as the slowest device on the network, therefor if you are connected to a dsl router and you have a 1gbps very expensive nic, you won't get more than 100mbps out of it due to the router. In 99% of the cases an on-board nic is just as good, if not better than a 3rd party one!