You might try moving the AP to a more central position. Sometimes just moving it to a different location in the room it IS in will make a difference. If the building is made of brick, stone, or steel, signal strength will drop rapidly as the singal passes trhough walls. There are repeaters for “G” frequency AP’s such as this:
I don’t know of any N freq repeaters…yet.
You might actually gain more through put by switching to G freq and using the repaeater. If a majority of your network use is for internet, then you will see little if any effect of switching from N to G.