In principle ethernet should be faster than USB, but only if it is a 10/100 card, as a 10Mb/s card cannot operate full duplex at that speed.
I'm a bit confused, though - you are only trying to get the one PC online, for now, aren't you? To connect both you will need two cards in the host machine & enable internet connection sharing.
Assuming it is just the one PC & a straight through, not crossover cable, it should simply be a case of turning it all off, connecting the network card to the modem, turning the modem on, leaving it 30 secs for the sync & ready lights to stabilise, then turning on the PC - it should pick up an IP.
<edit> I see Rob C has covered that already
