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On the assumption that your current network card is already a 10/100, a Gigabit card won't get you a faster internet connection.
Only implication from a cable point of view, since you use a modem, that I can think of is the need to power off the modem when you switch over the NIC.
Of course there may be software on your own PC that is expecting a specific NIC mac addy. I've seen this on a work server where I replaced a card and horrible things like DNS then didn't work properly.