Matched Set of Six 4GB, PC2-6400, DDR2-800, 800Mhz, 5ns Cycle Time, ECC, Fully-Buffered, 256x72, 1.8V, 240pin FB-DIMMsThis kit contains two matched FB-DIMM modules, which are specifically designed and SPD configured for the Mac Pro. This module is compatible with the latest Apple Firmware.These FB-DIMMs include full sized Apple spec heat sinks. They conform EXACTLY to the Apple original thermal spec.Configuration Information:The Mac Pro 1,1 and 2,1 uses a type of memory called an FB-DIMM. There are eight (8) slots, and the FB-DIMMs are installed in pairs (2x at a time). In order to run 256 Bit Addressing modules are installed in two matched pairs (4x identical modules). The original 1GB is installed as a pair of two 512mb FB-DIMMs. The system maximum is 32GB with the installation of eight (8) 4GB FB-DIMMs. Heat Sinks: MacMemory Net uses the only heat sink certified by Apple for use in the Mac Pro. Other brands use heat sink types that cause overheating leading to increased fan usage and slower performance. Our heat sink runs 7 degrees cooler than the folded plate design, and 15 degrees cooler than the flat spreader design of heat sink. Matched Modules: MacMemory Net kits contain a matched set of DIMMs. The kit consists of identical brand/model/spec IC's, with matching batch numbers. Our memory will also mate perfectly with Apple original DIMMs.