I have two clients running this processor with 512MB of RAM and windows 2000 that runs the AJAX web client just fine running FF3 with 100Mb NIC. My home computer was, up until 2 weeks ago, running the 3000+ Athlon across the Internet and it ran just fine. As a matter of fact, one of my Zimbra servers runs on a 3000+ Athlon. These Barton core processors have plenty of power to run the AJAX client.
Windows 95 and 98 era machines are probably running considerably less hardware I would think.
Have you checked all machines for viruses and/or trojans? These can kill a browsers performance quicker than anything.