Sorry for a simple question. If I change my wireless ADSL router from a G series to an N series to obtain a greater range (distance), do I need to change the wireless card in my laptop?

You shouldn't need to. You will probably have to run the router setup to change the device being controlled.

Will I benefit from the extra range and speed that the n series gives or should I invest in a n series usb wireless adapter?

You may find that the existing router has the N and G capabilities.

Will I benefit from the extra range and speed that the n series gives or should I invest in a n series usb wireless adapter?

No. If you see any difference in range it will only be by chance due to different antenna orientation of the new router and NOT because it is 11n.

Your router will likely support 11g (not all do) but if your laptop adapter does not support 11n, then you will not see any advantage using a 11n router. And note unless you get a "Simultaneous Dual Band" router (the most expensive), your router will limit the wireless performance of the other 11n devices on your network while communicating with any 11g devices. So your best bet is to buy a 11n adapter for all your devices and run full 11n across your wireless network.

Note that, IMO, this expense is easily justified because 11n is a much more secure protocol and I recommend all wireless networks run 11n.

Many thanks for your comprehensive reply. Common sense really but I appreciate the views of the D-A-L gurus. Thanks again.