I am currently connected to my router via a 100 megabit cable (the router port itself is either 100M or 1G, I can't remember). There are no free ports on the router, but I want to connect additional computers to the network. If I get a gigabit switch and connect two of my PCs to it using gigabit cable, can they communicate at gigabit speeds, or will they be limited by the 100M connection back to the router? (In other words, does traffic between two computers on the switch have to be sent through the router, or not?)
Yes, all computers connected to the gigabit switch can transfer data faster to other gigabit connected machines as long as the computers have gigabit Ethernet adapters. I generally get around 885Mbps top speeds due to network overhead. Anything that goes through the fast Ethernet (100Mbps) router, however, will be bottle-necked to 100Mbps. I would connect as many of the machines as possible that have gigabit adapters to the faster switch if you transfer data between them.