I would like to add certain points to this discussion. I have made a detail study on this and I found that if you have a wireless N router and other devices are of G then this is sure that you will not gain any extra coverage or range.
But let us say you have G router and it gives you internet access up to 35meters with 54mbps then there is no surety that you will get 54mbps speed at 35meters. It could be lesser also at that point. But if you have N router but the other devices are of G then it is assured that you will get 54mbps at 35meters also.