The all use 2 dual core Xeon Woodcrest processors and according to Apple are $800 cheaper than a similar Dell configuration.
Two 2GHz, 2.66GHz, or 3GHz Dual-Core Intel Xeon 5100 series processors
Intel Core microarchitecture
4MB shared L2 cache per processor
128-bit SSE3 vector engine
64-bit data paths and registers
Lower power optimization
1.33GHz, 64-bit dual independent frontside buses
There are 3 chices of cards:
NVIDIA GeForce 7300 GT with 256MB of GDDR2 SDRAM, one single-link DVI port, and one dual-link DVI port
ATI Radeon X1900 XT with 512MB of GDDR3 SDRAM and two dual-link DVI ports
NVIDIA Quadro FX 4500 with 512MB of GDDR3 SDRAM, two dual-link DVI ports, and one stereo 3D port.
You can install internally 4 Hard Drives to have up to 2TB
667MHz DDR2 ECC fully-buffered DIMM (FB-DIMM) memory
Eight FB-DIMM slots on two memory riser cards (4 slots per card) supporting up to 16GB of main memory
Up to 256-bit wide memory architecture
Here are the specs for them
http://www.apple.com/macpro/specs.html