The head node consists of:

Motherboard should support RAID and CPU

CPUs
Intel Core i7 920 Nehalem 2.66GHz 4 x 256KB L2 Cache 8MB L3 Cache LGA 1366 130W Quad-Core Processor

System Memory
2 GB RAM for the head node

Hard Drive
4 1.5 Terabyte Drives RAID

Motherboard
ASUS P6T SE LGA 1366 Intel X58 ATX Intel Motherboard

Graphics Card
Something cheap

Each node consists of:

CPUs
Intel Core i7 920 Nehalem 2.66GHz 4 x 256KB L2 Cache 8MB L3 Cache LGA 1366 130W Quad-Core Processor

System Memory
OCZ Gold 6GB (3 x 2GB) 240-Pin DDR3 SDRAM DDR3 1600 (PC3 12800) Low Voltage Desktop Memory Model

Hard Drive
Western Digital Caviar Green WD6400AACS 640GB 7200 RPM 16MB Cache SATA 3.0Gb/s 3.5” Internal Hard Drive (bare drive)

Motherboard
ASUS P6T SE LGA 1366 Intel X58 ATX Intel Motherboard

Graphics Card
EVGA 01G-P3-1180-AR GeForce GTX 285 1GB 512-bit DDR3 PCI Express 2.0 x16 HDCP Ready SLI Supported Video Card