Airport connects to router, but no internet

Hi.

I've recently gone back to my iMac G3 400 DV SE and upgraded it with extra RAM, OS X 10.3 and an Airport card.

Everything is fine, except I can't get onto the internet. The Airport card connects to my router, but as soon as I open Safari, I am greeted by a message that tells me that the server could not be contacted.

Any help and guidance on how to correct this problem would be most appreciated, as my router lives in the downstairs hallway and the iMac lives upstairs in the study!