Get another router, then install DD-WRT firmware on the WRT54GL and run it as a wireless bridge. Being that the WRT54GL is only a "G" router, I don't know how having a "N" router will really help you. I'd probably get another WRT54GL and put DD-WRT on it too and find the best channel to avoid interference, and crank up the power to somewhere between 70 and 100 mw.
Wireless Bridge - DD-WRT Wiki