Architectural traits reaching back to Pentium remain present in the Intel-powered servers of today. The limitations of those servers aren't likely to be noticed as long as the routine of IT and commercial server buyers is to add capacity by scaling out, purchasing new two-socket servers. But the time will come when adding a rack server, or a rack of servers, is no longer the wise person's path to increased capacity. Smart planning will lead you to handle bigger workloads without more servers.
The terms "scale up" and "scale out" are sometimes unfamiliar to x86 buyers. They refer to the locale of capacity expansion, computing ("thinking") capacity in particular. A server that scales up can be made to handle substantially higher workloads through upgrades inside the chassis. These systems cost more at first, but they're designed to have untapped capabilities that you can turn on with an incremental investment far less than that of a new server.
Scale up is the factor that has kept proprietary Unix big iron in business. Linux on a commodity two-socket Intel server was supposed to push HP, IBM, and Sun out of business. It looks that way if you see a rack chassis as a rack chassis without regard for what's inside. But scale-up maximizes everything from power savings and server consolidation ratio to server longevity, with the bonus of lower long-term costs and higher availability. All AMD Opteron servers scale up. It's baked into the CPU, the bus, and the total system architecture. AMD's strategy is to make it possible to scale up any Opteron server for five years with only a CPU swap, no new server required. This stands in stark contrast to Intel's "tick tock" plan that attempts to nail IT to the stereotypical two-year purchasing cycle. Intel's two-year cycle of obsoleting chips makes parts scarce and expensive, so that if you do buy an Intel-based server with empty sockets with plans to scale it up, it's unlikely that CPUs precisely matching the models you have now will be available, and the availability of FB-DIMM memory at your existing Intel servers' speed may be rare as well. AMD's five-year plan is more in line with the way IBM treats, and retains, its customers.
Scale out means bigger racks, more servers, more heat, higher power and cooling costs, another tick on your service contract, another hand to hold in the middle of the night, and so on. The only thing going for it is convenience, and that's a powerful motivator. Most shops have the deployment of new rack servers down to a science, and there's rarely a need to even remove the cover on a server before you slide it into the rack. Opteron servers yield to the very same plug-and-play initial deployment, but in a few months when you'd ordinarily add a new server, you can take the scale-up route of your choice: Swap out your Opteron CPUs with higher speed or more cores, add RAM or use faster RAM, or fill empty CPU sockets with new CPUs. It really is as simple as it sounds, and when you (or your field service person) buttons up the case, you have a new server, or two, or two and a half, where your two-socket server used to be.