Looks like you max on the Athlon Thunderbird cores that run on 266 FSB.
Here are some possibilities, since your not going to find these "new" (more than likely:eBay listing
' ">http://cgi.ebay.com/ws/eBayISA...0&rd=1&ssPageName=WDVW1.4 T Bird
' ">http://cgi.ebay.com/ws/eBayISA...6&rd=1&ssPageName=WDVWAnother 1.4 T Bird...
This should give you an idea of how much you are looking at to upgrade your machine.
Hope this helps,