Sandy wanted to run a computer and credit card swipe terminal on the same ethernet connection. Leo says that may not work since the credit card terminal needs it's own connection to the net. It requires a unique address. A router could help because it would route the traffic and it would be a different device address internally. Will it slow down her computer? Leo says a minuscule amount. Nothing that Sandy would notice, though. Any cheap router will work for this, but Leo likes D-Link.
Since Sandy is actually connecting to a router already, she will need a switch instead, which D-Link also makes. Netgear is another good option.