Odds are, you'd get more mileage out of connecting your PCs via a true local area network. NICs are distinguished from each other by their support for differing network protocols, bandwidth, and cabling. For this article, I'm going to assume that you want a card which supports the Ethernet protocol. A NIC's bandwidth determines how quickly it can send and receive data across the network. Some cards accept only twisted-pair cable, which has a connector that looks sort of like the one that connects your phone to the wall.
Some cards accept both types.
Coax cabling will limit your network to speeds of 10Mbps. You don't need to use identical network cards in each machine. Still, if you have a brand X card in one machine, and a brand Y card in the other, they should still be able to communicate. For this option, you need a network card for each PC and a special cable to connect them, called a crossover cable. A two-station LAN most likely doesn't need the extra speed offered by 100Mbps cards.