This is the 64 bit edition of CentOS version 5.4 and it(x86_64) supports both AMD64 and Intel EMT64 architectures. CentOS 5.4 is the fourth update to the CentOS 5 distribution series. You can read the release notes at CentOS.org

About CentOS

CentOS is a community-supported, free and open source operating system based on Red Hat Enterprise Linux. It exists to provide a free enterprise class computing platform and strives to maintain 100% binary compatibility with its upstream distribution. CentOS stands for Community ENTerprise Operating System.