You are replacing a device designed to connect to a wireless network with a device designed to create a wireless network, these devices have different functions.
As to your first question, leave the G drivers installed. They are mapped specifically to the USB card and will not interfere with any other wireless adapter.
Your new device is a Router, if you already had a wireless network up and running then this device would be an upgrade to your existing wireless Router, not you USB network card.
You would need to connect the Linksys N600 to your broadband modem (removing your existing Router) and then set it up to create and manage your wireless network, then you would continue using your USB “G” card or buy a new USB “N” standard card to take advantage of the increased speed in the Linksys N600.
If your current broadband modem is also your wireless Router then you would have to turn off the wireless function, plug the new Linksys N600 into the modem with a network cable (Using one of the Linksys N600 standard ports, not the colored one) and set up the Linksys N600 as an “Access Point” when the question is asked.
This is a good Router, you will be pleased with it if you set it up properly.
What is happening now is your Linksys N600 is “seeing” your Network but for it work to without removing your existing wireless Router you must set it up as a “bridge”, this is possible but probably pointless unless your goal was to increase the wireless range.