Sign on to .mac
Click on Account (top item in left side panel)
You may be asked to log in again (security)
In buttons on right, click on Credit Card Info
At bottom, set Auto Renew to off (unclick)
You're done
Original Airport cards for G3 iMac's are hard to find and expensive. Go with an external USB wireless adapter such as the Belkin F5D6050 or the D-Link DWL-122. I used the Belkin and it worked fine. Paid less than $20US for it. As for a wireless router, I've used the D-Link DI-524 and the Belkin...