Symphonic Orchestra and other sample-based instruments aren't going to work the CPU too much. They're memory- and disk-intensive and so you should have plenty of the former and a fast hard drive if possible.

Keep in mind that the "slower" processors in a certain line are the ones which failed certain tests in QA. That doesn't mean they're defective or anything, just means that they might have had to disable some of the L2 cache or lower the clock speed to get it stable. When they test the chips in the lab, they're looking at them through a microscope (figuratively) and so you may never notice the problems that they found at higher clock speeds. On the other hand, you might. It's a gamble, and what works fine for one E6600 may not work for another E6600 because it's all based on an imperfect manufacturing process.

Some people believe that AMD and Intel deliberately underclock a bunch of chips, just so they'll have an artificial "midrange" product to sell (as in a type of market segmentation). I don't work for Intel, but I find that argument economically unsound. It's possible that they just don't bother to test the majority of chips at higher clock speeds, because the tests are expensive to run and they figure that only a limited market will buy those anyway, but that's pure speculation and there's absolutely no evidence in favour of it.

So overclocking is fine, in theory, but it's really something for enthusiasts who enjoy tweaking their PCs, whereas most of us just want to get on with our work and would prefer the guarantee of stability. As for me personally, I'm running everything completely passive so I'm not even going to try to OC.

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My party schedule:
2009-02-21 - DJ Attention @ I'm So Popular
2009-06-18 - DJ Annoying @ People Need To Know Where I'll Be
2012-11-32 - DJ Insufferable ɸ Or At Least the Stalkers I Complain About
2048-06-66 - Spastic & Whocares ¶ Although I'm Actually Flattered
9999-45-81 - Tweaker Gimp ☼ I Probably Won't Even Go To This But I Have To Make Sure I Fill Up All The Available Space Here