The Mac Pro "Eight Core" 2.66 (Early 2009/Nehalem), is technically classified as "made-to-order" configuration of the Mac Pro "Eight Core" 2.26 (Early 2009/Nehalem), but is also listed as a separate model for easy reference. Apart from the speed of the processors, both models share identical specifications.
The Mac Pro "Eight Core" 2.66 (Early 2009/Nehalem) features two 2.66 GHz Quad Core 45-nm Xeon X5550 (Nehalem) processors with a dedicated 256k of L2 cache for each core and 8MB of "fully shared" L3 cache per processor.
The standard configuration featured 6GB of 1066 MHz DDR3 ECC SDRAM, a 640 GB (7200 RPM, 16 MB cache) 3Gb/s Serial ATA hard drive, an 18X dual-layer "SuperDrive" and an NVIDIA GeForce GT 120 with 512MB of GDDR3 memory.
Expansion includes two external 5.25" "optical" bays (one available by default), four internal 3.5" "cable-free, direct attach" hard drive bays (three available by default), and four PCIe 2.0 slots (one free PCIe 2.0 x16 slot and two free PCIe 2.0 x4 slots with the default single graphics card installed).
Port selection includes five USB 2.0 ports, four Firewire "800" ports, dual Gigabit Ethernet, and both a Mini DisplayPort and a dual-link DVI port, and more. Bluetooth 2.1+EDR comes standard, AirPort Extreme (802.11g/n) is available as an option.
While the "Early 2009/Nehalem" models appear nearly identical on the outside as the "Early 2008" models, significant technical improvements were made. The 64-bit "Nehalem" architecture is considerably faster and supports "Hyper-Threading" -- which "enables simultaneous execution of two threads on each core" (so MacOS X identifies sixteen virtual cores on this model) -- and "Turbo Boost" -- which "dynamically increases processor speed according to workload demands" (so if an application is only using one of the eight cores it automatically increases the speed of the core in use and deactivates idle cores).
It also supersedes the frontside bus with a new "QuickPath Interconnect" (QPI) system characterized as a "bidirectional, point-to-point connection" that provides "quick access to the disk, I/O, and other Mac Pro subsystems".
Additional improvements include four PCIe 2.0 slots (instead of two PCIe and two PCIe 2.0 slots), four Firewire "800" ports (instead of two Firewire "400" and "800" ports), and a revamped internal chassis with the processors and memory on a removable tray assembly.