Gigabit switches will still function at Gigabit line-speed even if there is a 10/100 only device connected to the switch. Even if your router is only 10/100, you can still buy a gigabit switch and get Gigabit connectivity. In this situation, attach all Gigabit devices to the Gigabit switch, then connect the Gigabit switch to the router. Gigabit devices are only limited to 10/100 speeds if they are communicating with a 10/100 device.