The special thing with this project is that I use a powerline network adapter to supply internet to the wrt54g. I know I can configure it as a repeater but I prefer to use it as a native router/access point and because at the roof I have a power outlet and I use the home electric instalation as a LAN for all my computers, networked harddisks, printers and media players and of course for my home server.
I bought this Nema box with Power outlet for $5 at a garage sale and the wrt54g router and powerline adapters for $12 at goodwill, so I can say that the total cost of my proyect is below $20 USD, because the Cisco/Cushcraft 5.2dBi antenna was given to me by a friend at no cost.
I replace the original linksys firmware of the WRT54g with "Tomato firmware" to be able to increase the strenght of the WiFi signal from 70mW to 245mW and to have a better MAC filtering and of course to reduce heat in the enclosure (I live in a desertic region) I strip the router and leave just the router's motherboard.
You can leave the original antennas to the router and have good wifi coverage around, but as I have this external antenna on hand I use it.
I think is not necessary to post all the steps to make something similar as this was a very easy project. Please note that powerline networking is a standarized technology and almost all powerline adapters work with others no matters the model or make.
You can download Tomato Firmware and check update instructions at this website: http://www.polarcloud.com/tomato
Also you can install DDWRT to this kind of routers (www.dd-wrt.com)