If the broadband you receive is slower than what the modem can handle then can continue to use that.
The utility of gigabit in the home comes when you start streaming video between machines on your own network.
To avoid the modem being a bottleneck, just take one port from it to connect to a Gigabit switch.
Wireless - best bet is to situate a G/N wireless access point in the room where you will be using wireless, to give the best possible speeds.
With the kit you have now - you have two routers, which may complicate things for people who are encountering networks for the first time, hence why I suggest to add faster wireless buy a wireless access point.