This relatively new addition (10/02) to our group is a custom-built 22 processor cluster by Aspen Systems. Each node consists of dual AMD/athlon processors (2000+) -- 10 are compute nodes with 3GB/ea of DDR RAM and an aggregate (11 nodes) of ~750 GB of Ultra160 disk storage. Each node communicates via Myrinet, a high-speed, low latency network with job queueing via PBS Pro. Our group predominately uses the MOLPRO program using mpich over myrinet.
We've also recently installed a new 10 processor cluster. Each node consists of dual AMD/Opteron (2.0 GHz) processors with 8Gb of RAM and 140 Gb of Ultra320 SCSI disk space. Communication between nodes is currently handled by Gigabit ethernet.