The so called "Big Mac" machine uses dual 2.0GHz Apple G5 systems, while the Dell Poweredge system at NCSA uses Pentium 4 3.06GHz chips. In fifth place is a machine using the Itanium 2 at 1.5GHz installed at Pacific Northwest National Labs. AMD is in at number six, with the Lightning system at Los Alamos National Labs and which uses 2GHz Opteron systems.
You can find the rest of the list here. ยต