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One of the most interesting additions to Nehalem was Simultaneous Multi-Threading (SMT) or Hyper-Threading as Intel calls it. This is a technique that enables a single physical core of a CPU to act as though it were two, thus improving multi-threaded performance. While the gain isn't as good as it would be if you added another whole core, it does provide a noticeable boost in performance and uses a lot less silicon.
Now, it's with Hyper-Threading that Intel has made the whole Core i* number scheme rather complicated. Instead of calling Bloomfield, Core i7, and Lynnfield, Core i5, (and thus making everyone's lives easier as we wouldn't have to constantly be referring to internal codenames) Intel has given Lynnfield CPUs with Hyper-Threading the Core i7 800 name and those without Hyper-Threading the Core i5 700 name. Thus we now have to retrospectively call Bloomfield CPUs Core i7 900.
The last thing to note about Lynnfield on an architectural level is its improved Turbo Mode. Turbo Mode was introduced with Bloomfield and it refers to the CPUs ability to dynamically overclock itself, by adjusting its multiplier, depending on workload. When enabled, the CPU will constantly try and work at its maximum speed which, if all four cores are under load, will likely be the CPU's reference speed. However, if only one or two cores are under load then these cores can be made to run faster, via overclocking, without taking the CPU higher than its thermal envelope (TDP). In other words, while your CPU may be nominally rated at 3GHz, when only one core is working it could be running at 3.33GHz, say.
Now, Turbo Mode was present in Bloomfield but its impact was minimal because it could only increase the multipler by 1 (which when you're starting with a multiplier of 20, is quite a small change). However, with Lynnfield, Intel has really opened the taps on Turbo Mode, allowing the multiplier to be upped by 4. Specifically, the three CPUs that are being launched today are clocked at 2.93GHz, 2.8GHz, and 2.66GHz but have max single-core Turbo Mode speeds of 3.6GHz, 3.46GHz, and 3.33GHz respectively - not bad for a free performance boost.
One last thing to note about Core i5 CPUs is that they don't have Virtualization Technology for Directed I/O (VT-d), a recent enhancement to Intel's hardware virtualisation support. This should be of little concern to the vast, vast majority of home users though.
So that's the theory out of the way. Now, let's take a look at the chips and boards themselves.