You will still be able to connect to the 802.11n router, but at the 54Mbps speed of 802.11g. In practice, the actual speed of your adapter will be closer to 30Mbps so you won't achieve the full benefit of a 50Mbps broadband connection.
Depending on the hardware, 802.11n devices will communicate at 150Mbps or 300Mbps.
I recently bought one of these click here extremely small 150Mbps "n" adapters at a very good price. It works well.
If sticking with your Belkin G adapter, make sure the router is set to connect with both "G&N" adapters.