64 bit AMD machines really shine under linux. I have an apache/mysql system at home running on a pair of opty 248s. They are literally 40-50% faster overall for my database apps than a Dual Xeon 3GHz. The memory bandwidth of A dual Opteron system is simply staggering, and when code is compiled for 64 bit it makes a very noticable difference. For anyone interested, this server is running Gentoo 2004.2/AMD64 with compiler flags -O3 -march=k8 -pipe -funroll-loops -fomit-frame-pointer -fpie -fstack-protector.
I have another system (An Athlon 64 3200 socket 754) running Gentoo/AMD64 that is my HTPC (it runs freevo). It used to be hard to get freevo working on AMD64, but now it's easy as they are finally putting ~amd64 and amd64 into the ebuilds for everything. The Athlon 64 plays back hi def DVDs with ease whereas my old P4 2.53 had problems with 1080p video playback if it was in windows media format.