If you add the AirPort Extreme, you will need to keep the Netgear modem/router in place, since the AriPort Extreme does not have a modem on board, and you will need a modem if you want to be able to access the Internet.
If you decide to add the AirPort Extreme, in general, your network will look like this:
Internet Connection >>> Netgear Modem/Router >>> AirPort Extreme >>> Devices
So, you will in effect have two routers on the network. Unless you feel that the AirPort Extreme will offer better performance....or you simply like the look of the Apple product....you probably don't need two routers. But....if you pull the Netgear....you will need another separate modem to work with the AirPort Extreme.