Copy pasta, since I couldn't have said it better myself:

So the 3.1 GHz Sandy Bridge is 23% faster than a 2.8 GHz Nehalem. So roughly 10-15% faster clock for clock as Anand says in the teaser intro on the main page. Also, you get 10% less power. Not bad. However, that means that Bulldozer only has to slightly perform better than Nehalem to match Sandy Bridge clock-for-clock and live up to the 10-100W power envelope.

I change my vote on the weekly poll to match Sandy Bridge performance.
Be careful when comparing clock-for-clock. In the comments to the article, there was this exchange:

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"I take it turbo was also disabled on the rest of the parts used to compare, right?"

"Turbo was enabled on everything else - SB performance should be higher for final parts.

Take care,
Anand"
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The Nehalems are tricky: given half-way decent cooling (which I'm sure AnandTech has), they will Turbo even when all 4 cores are active. Thus, the supposedly 2.8 GHz Nehalem actually runs at a minimum of 2.93 GHz and goes up to 3.33 GHz in single or dual threaded tasks. A lot depends on what Turbo scheme Intel has chosen for Sandy Bridge.