The first AMD64 processor on the market was the Opteron, which is geared toward high-end workstation and server machines. The Opteron is a 940-pin socket processor with an integrated heatsink, a 128-bit wide memory controller on-die (which is roughly equivalent in theoretical memory bandwidth performance to a dual-channel memory controller in a 32-bit Athlon XP system), the ability to use 144-bit registered ECC RAM, and some models are capable of working in tandem with up to seven other on-board processors in a multi-CPU machine. As of this writing the Opteron has three distinct models: the 100, 200, and 800 series and they can do single, dual, or up to eight-way processing, respectively. Each model performs similarly, with the only significant differences between them being multi-CPU capabilities and the number of HyperTransport links each has. More HyperTransport links means increased data throughput between the processor (or processors) and the devices mentioned above in the HyperTransport section. The Opteron requires registered memory and does not support unregistered RAM at all. Opteron machines can use either registered ECC RAM or registered non-ECC in a variety of sizes and chip densities, although you will be limited to a smaller amount of total RAM with 64-bit (as opposed to 128-bit) DIMMs.