|if you are using your G router at 54mbps or N router at 300mbps|
I don't know about that statement. I don't know of any SOHO level equipment that gets 300 Mbps. 100 Mbps, yes.....
As stated if you do the math you'll see the following:
Wireless G = 54 Mbps
Wireless N = 100 Mbps
Internet = 7 Mbps
Which do you think is fastest? Slowest?
LAN rates will far exceed your WAN connection rate