I have setup an Asus RT n16 with 3 computers that support gigabit ethernet and a media server on one floor. On the other floor I have set up a fast ethernet switch (connected to my asus router) which is connected to several fast ethernet enabled computers.
Would the fast ethernet devices slow down the entire gigabit networking to 100mbps connections on my floor - similar to how a wireless n network running at 300mpbs must slow down to 54mbps when a wireless g device is connected to it?
Although you can connect two computers directly to each other via their Ethernet ports with a crossover cable, larger networks need a switch, which is frequently incorporated into a router. The network runs at the speed of the slowest component, so if you use a switch that runs at a slower speed than the network clients, the clients connected to that switch will run at that slower speed
Your gigabit devices will autonegotiate the link speed so if you plug it into a fast ethernet switch, it should auto negotiate to 100Mb/s. If you had a gigabit switch, then of course it should auto negotaite to 1000Mb/s.