Buy a Linksys WRT54G (~$40) and load DD-WRT on it. Then configure it to be a client bridge, like this: http://www.dd-wrt.com/wiki/index.php/Client_Bridged
This will just give you 4+ wired ports off of one wired connection. You would need another radio (basically a second router) to make your own wifi network off of that connection.
The real question is, why would you want to do this? What is the point of receiving the wireless signal and then re-transmitting it? It's going to create a lot more noise on the 2.4ghz spectrum than is necessary.