Hi,
FYI there is now a working Debian system available for AMD64. Setup is still a bit non standard, but it is quite stable and complete (> 88 % of Debian packages). For what Wikipedia would need for its servers, MySQL, PHP4, Apache, Squid are available. It is a pure 64 bits environment.
http://alioth.debian.org/projects/debian-amd64/ and the doc http://alioth.debian.org/docman/view.php/1314/21/debian-amd64-howto.html
Yann