Send messages in plaintext - it's significantly smaller. Otherwise, you have a very good question. Correct me if I'm wrong, but the amd64-k8 kernel is the one you're looking for, since the Athlon64/Opteron is of the k8 generation of processor. I can understand your need to optimise your experience - just be careful, as not all of the apps you'll find in the i386 distribution are on the scene in the amd64 version!