Check if your firewall is supported

http://www.dd-wrt.com/wiki/index.php/Supported_Devices

read the install notes

http://www.dd-wrt.com/wiki/index.php/Installation

Install directory

http://www.dd-wrt.com/dd-wrtv2/downloads.php

need more info. Check it out on Wikipedia

http://en.wikipedia.org/wiki/Dd-wrt

note the difference  going to the Mini-WRT

DD-WRT download forum

cheapest place to get WRT-54GL