Paying for 50 mbps down speed but only getting 20-30 mbps in living room, and 5-10 mbps in bedrooms.
The first thing to say is that it's a good idea to run several different speed tests using different servers before you make judgements about any steps you need to take to improve things.
I have a BT Infinity connection here, in London. Testing on the company's server showed a downstream speed of 72 mbps and an upload speed of 15 mbps.
Then I used Comcast's test on the Washington DC server. That gave me 60.8 downstream and 8.7 upstream.
The speed difference between your living room and bedrooms could be down to one or more reasons - the wireless network speed (dictated by your router), intervening walls, and the number of people using the connection at any one time (contention).
You can't improve the speed on the inbound line, but your modem/router combination is what will make a difference. Your current modem can handle a maximum of 30 mbps in each direction. The Netgear CGD24G can act as a range extender with its built-in wireless access point, and will support up to 54 mbps, so that's worth a try.