Upgrade your linksys WRT54G version 8 to DD-WRT and make it awesome
First follow these instructions for getting dd-wrt: Get Linux running on your Linksys Router (this is for version 8 of WRT54G. You can check the version by looking at the bottom of your router. i.e. ver 
Then boost your wireless range: Go to http://192.168.1.1 user: root password: pass. Click Wireless > Advanced Settings. Change TX Power to 70. Click Save and the click Apply Settings.
Next let’s extend the range of the network using an apple airport express.
Wireless > Wireless Security > WEP
Default transmit key: 1
Encryption: 128
Passphrase: whatever you want
Save the first key in a text document for later
Apply Settings
Then follow these instructions - How to setup WDS between a wireless router and an Airport Express. Making sure you have your linksys router connected to your dsl box, and then your apple airport express connected to your linksys router.