The Xserve 3,1 DDR3-1066 comes in two versions, the 4-core and the 8-core. The 4-core has 6 DIMM slots, and it upgradable to a maximum of 24GB with the installation of 6x 4GB modules (2x 12GB kits). The 8-core has 12 slots, and it is upgradable to a maximum of 48GB with the installation of 12x 4GB modules (4x 12GB kits). Since the new Xserve runs a "Tri-Channel" memory interface, memory MUST be installed in matched sets of three at a time.
Matched Modules: The Xserve 3,1 is based on the Intel's Xeon 5500 Series. It runs on a Tri-Channel memory interface. The Ramjet kits for the Xserve 3,1 DDR3-1066 contain a matched set (3x) of DIMMs. The kit will consist of identical brand/model/spec IC's. They will be from a single batch, and will have matching series numbers. This ensures maximum performance, and perfect compatibility.
Please view Apple's installation guide for more information.
Apple's installation guide.