I don't think bridging is the correct term you want, but a router will be better than internet connection sharing. With internet connection sharing, videos on your apple tv may lag when your computer's CPU is under load. With a router, your devices will not be dependent on each other, although one device can steal bandwidth away from another device. If the router has QoS (quality of service), then you can make video a higher priority.
If the router has gigabit ports on it, they will automatically negotiate the correct speed, gigabit, 100 Mbit, 10Mbit.