Conclusions
We'll tackle this from the top down, starting with the Core i7-975 Extreme at the peak of the market. Obviously, this processor isn't intended to be an astoundingly good value or anything like that, but boy, you will be getting an awful lot of CPU for your money, regardless. Intel has placed the performance crown a long way out of reach for AMD, and that isn't likely to change any time soon, especially with the debut of this new model at the same price as the 965 Extreme before it.

These days, many folks don't feel the need for a faster processor quite as acutely as they did back when we started doing these reviews, nearly ten years ago—and rightly so. PCs are often fast enough for everyday tasks, and even the latest games don't take great advantage of, or tend to require, more than two CPU cores. There's a legitimate question to be asked about who really needs a processor like the Core i7-975, with four cores, eight hardware threads, and three channels of DDR3 memory. But if you're running a seriously performance constrained application that chews up loads of time—be it video editing, scientific computing, or just compiling a large software development project—you're probably acutely aware of the fact that more speed would be nice and that the Core i7-975 Extreme's thousand-dollar price tag is, compared to the time you might save with a faster CPU, just a drop in the bucket.

Such applications don't usually mix well with extreme overclocking, but the 975 Extreme's potential to reach 4GHz with a minimum of effort makes it a heckuva nice plaything, as well. Just think: at a 4GHz core clock, it's hitting 4.26GHz in single-threaded applications via Turbo mode, which helps explain the ridiculous performance we saw out of it. If the Extreme Edition's job is to be the aspirational performer, the top-of-the-line exotic that makes you want to buy something related at a more affordable price point, well, it plays that role awfully well. I'd suggest a Core i7-920 plus a little overclocking as the sensible alternative, for what it's worth.

The next sensible step below that may be the Core 2 Quad Q8400, which acquitted itself reasonably well in our tests. At 2.66GHz, the Q8400 is fast enough to play games and handle most single-threaded applications without issue, although in such cases, it was oftentimes slower than value dual-cores like the Pentium E6300 that offer higher clock speeds. I suppose my grumpiness over "value quad cores" has been largely blunted by the Q8400's 2.66GHz frequency and its rather ample overclocking headroom (at least in our chip). Let's put it this way: I wouldn't pay more for a Core 2 Quad Q9400, which is the next step up the ladder, because the extra cache isn't worth the extra cash. The Q8400 is the better value.

The Phenom II X4 940 outperformed the Q8400 by a small margin in our overall performance analysis, so AMD may have the superior offering here. We only wish AMD didn't charge 30 bucks more for the Socket AM3 version of that same part, which is known as the X4 945. Also, if you're a gamer, you'd do better to go for a Core 2 Duo E8500 at the same price as the Q8400. You'll step up to 3.16GHz and a larger cache, and you won't miss the two cores in any of today's games (and probably few of tomorrow's).

But for the all-around PC enthusiast with diverse needs, the Q8400 is a solid option.

Of the three value dual-core processors, the Phenom II X2 550 is the obvious choice. Both the Athlon II X2 250 and the Pentium E6300 are reasonably attractive cheap CPUs, but the Phenom II X2 550's combination of a much larger cache, generally better performance, and low-effort Black Edition overclocking makes it easily worth the extra $15 or so over the other two options. The fact that it's a cut-down quad-core part doesn't sting in the least; even its idle power consumption is fairly low. I didn't expect to find myself saying this, but I think the gimpy quad-core will be the best option for most folks. I suppose that's why we test these things.