[LudwigVan]

Found this on MacWorld's site:

Does the Mac Pro use the same RAM as other Intel Macs? No. While the DDR2 RAM in the Mac Pro is the same speed as that in all other current Intel Macs (667MHz, compared to the Power Mac G5’s 533MHz), the Mac Pro uses Fully Buffered Memory modules, or FB-DIMMs. Each FB-DIMM has built-in error checking and correction, and carries a chip (known as an Advanced Memory Buffer, or AMB) that controls data transmission. As a result of the AMB chip on every RAM module, each Mac Pro DIMM comes complete with its own heat sink. Apple says that by attaching a heat sink to every module in order to dissipate heat, it can keep fan noise to a minimum. Apple says it’s provided a thermal specification for the modules to the companies that are manufacturing Apple-branded RAM for these systems. (Since they can come from different manufacturers, RAM modules may vary in appearance depending on how their manufacturer chose to meet Apple’s specs.) Apple’s also pushing for its thermal specification to be adopted as a standard by the JEDEC standards body, which would make it easier for third-party RAM vendors to manufacture heat-sinked RAM to meet the needs of Mac Pro users. How do I install RAM in the Mac Pro? The Mac Pro has two small riser cards, each of which contains four FB-DIMM slots. To install RAM, you slide a riser card out and set it down on a flat surface. (The bottom of the riser card contains plastic feet so that the circuit board itself doesn’t make contact with the surface to place it on.) Then you install the memory modules—always in pairs of the same capacity from the same manufacturer. You need to install pairs of RAM in the proper order, which is printed on the inside of the G5 door: a pair in the top two slots of the top riser, then in the top two slots of the bottom riser; then in the bottom two slots of the top riser; and finally in the bottom two slots of the bottom riser. When you’re done installing RAM, you slide the riser card or cards back into the Mac Pro until they re-connect with the motherboard. Apple includes two 512MB modules in the stock configuration. As with the Power Mac, the Mac Pro requires matched pairs of RAM to function. To max out the RAM, you’d need to install 2GB modules in each of the Mac Pro’s eight RAM slots. That’s a lot of RAM.