Problem solved for me - thanks to Linksys
Having tried almost everything, including an Apple support case number and a contact, who never gave a feedback in spite of a promise that he would contact me, a solution of the problem with the slow Airport connection to my Wireless D-Link 614+ router has appeared - at least for me
After having send the support guy a total view of my system via Capture Data.app, which was mailed to me and the results returned, we had an agreement, that he would call me back at a precise time two days later, but he never did. (His name and email is known to me, and several calls to his fellow supporters, who promised to pass the message, and mails to him have only resulted in total silence).
Then came the Apple upgrade to 10.4.6.
Hoping this was the solution, it was installed, via Ethernet of course. Some improvements, but still very slow Airport connection.
But buying a new wireless router, Linksys WRT54G, solved the problem.
Airport speed from the Intel MacBook Pro is now fast - and compares by far to the fast speed of my old Powerbook.
It's a little extra money out of the pocket, but gives a better sleep and lower blood pressure instead of having to fight for an answer from a silent and no responding guy at Apple Support.