Opteron

64bit Extensions
The x86-64 bit refers to the fact that the processor will run old x86 code (which current AMD and Intel processors run on), but will support 64 bit extended words that allow for modified 64 bit code to run on Opteron. Microsoft recently announced their decision to support the 64-bit Opteron, which is a huge benefit for AMD.

This is AMD's attempt to slowly migrate the average user from traditional 32 bit processors into a 64 bit environment. The transition AMD is proposing is generally more well accepted than the Intel Itanium 64bit processor which is marketed more at large servers and workstations. (The Itanium will run old x86 code, however it will do so much slower than current Pentiums and is not recommended). The Opteron, however, gives the average user the ability to run both 32 bit and 64 bit code while the industry makes its slow transition over.

The 64 bit extensions are implemented on the Opteron core circuitry. There are also additional improvements on the chip that should greatly improve overall system performance. Specifically, these are the on-die DDR Memory Controller and the on-die HyperTransport Technology.

DDR Memory Controller
This is a portion of the chip dedicated to controlling access to memory. This is a drastic difference from traditional AMD and Intel chips. Traditionally the memory controller is located on the Northbridge and a function of the chipset. This new on-die DDR Controller allows the Northbridge to be simplified and lessens traffic on the Front Side Bus. The controller also now operates at the processor frequency, causing it to reduce memory access latencies.

While this initially seems like a huge benefit it suffers from a drastic fallback that if you change the memory interface (like from DDR 200 to DDR 400) you have to redo the memory controller, which means you need a new processor. AMD has included the option to turn off this memory controller that in on die so that chipset manufacturers can use their own Northbridge memory controller for different technologies.

HyperTransport Technology
This is basically a point-to-point link connecting chips. This theoretically will allow different Opterons to easily connect to one another, as in a multi-processor motherboard. It basically provides a quick, seamless, and integrated way for AMD processors to talk to one another. This will make it much easier to build multiprocessor machines.

Performance
TBD - Benchmarks were recently released and I will be incorporating that data shortly. Initial reviews seems to show the Opteron in a very favorable light, at least on some benchmarks. The waters are getting muddier and muddier as designers try to tweak the last bit of performance out of their designs they frequently optimize for one type of application but not another.