Not sure exactly about your config, do you have an ISP supplied router and then your own one plugged into that? If so DHCP needs disabling on the gateway one and the gateway IPaddress in yours set to the IP of the ISP one. (plus yours may need a static address. You can't have two devices running DHCP on the same network or it causes all sorts of bother.
We have this kind of setup with the second router as a pure wireless AP, keeps the kids stuff and ours separate, we have anywhere up to 15 devices running at once off a standard 40MB ADSL line without any issues, so 120MB should be plenty!