Even though there's an official AMD64 client for Linux, it refers to too many dynamic libraries and requires a fairly recent Linux setup to run on.
So, one more time, I'm making available the AMD64 Linux client here
. It refers to a minimal set of standard dynamic libraries whose version requirements should be satisfied by Linux systems up to 2 or 3 years old, however it was built with a fairly recent version of GCC, 4.1.2.
The drill's still the same:
The official AMD64 Windows client can be found here
For more information, see the BoincStats Forum