The speed of a cable modem is very slow compared to even a 10Mbps network card, actually, about one tenth( 1/10) the speed of a 10Mbps card and 1/80 the speed of a 100Mpbs card.
You do want a card that has low cpu utiliation.
My advice is to get a name brand card (Netgear, Linksys, Intel, 3com) that doesn't break the bank.