Yes, N speeds are much better than G and yes, you would configure the N router as an access point.
Connect the N router to the Uverse gateway with an ethernet cable - make sure you use one of the N router's LAN ports and not the WAN/Internet port. The router will be set to gateway mode by default, so you'll want to change it to access point mode. Depending on the router, the terminology will be different but it shouldn't be hard to figure out - each mode will have a short description that should be pretty self-explanatory, i.e. "This is the mode to use if the router is connected directly to the modem".
You don't have to, but you might as well disable the wireless on the Uverse gateway since you won't be using it. You'll have to set up a new wireless network on the N router, and your device settings would have to be changed to connect to the N network. Make sure you use WPA security at the minimum.
One caveat: If you have any devices that are only wireless G, then keep the G network on the gateway and connect the G devices to that network. The reason is that a G device can connect to an N network, but it will slow everything down to G speeds. This means that any device connecting to the N network should have a wireless N adapter.
I also have Uverse and this is the exact setup I use.