Ideally, you want to use identical network cards in all your computers. But if the boss's computer is down, you'd probably settle for whatever network card the corner network street vendor is selling today. That's why you should always keep at least one spare network card in the closet. You can rest easy knowing that if a network card fails, you have an identical replacement card sitting on the shelf, just waiting to be installed-and you won't have to buy one from someone who also sells imitation Persian rugs.
Obviously, if you have only two computers on your network, justifying spending the money for a spare network adapter card is hard. With larger networks, it's easier to justify.