If you are interested in learning why the AMD Opteron processor is currently the top dog in the X64 server space, you might be interested in the following Anandtech article:
http://www.anandtech.com/cpuchipsets/showdoc.aspx?i=1815&p=1
The article discusses the K8 architecture in depth, and explains why hyper transport rocks. I have been wanting to upgrade my Sun Ultra 10s to more modern hardware, and I think an AMD solution would be a good fit (especially since Solaris FMA now supports AMD CPUs!).