You can not really compare Core frequency of a i7 vs Phenom to get a ideal picture.
They work differently. But No, you will not downgrade.
The core i7 at 3.2 will be faster than a Phenom at 3.4.
Im in the process of buildiong a new machine myself.
It would seem if you already have a 1366 CPU, SB is not that big of a upgrade to justify the cost, for the most part.
If upgrading from something lower than 1366, SB is the best way to go.
The x6 on 1366 does have an advantage in ceratin areas. Such as highly threaded apps, but only if you use those apps alot to justify it. (And its expensive)
But for Quads in general, & everything else you may use it for, SB is just faster, and still can can even hold its own against the x6.
But if building new, I would just build Sandy Bridge TBH.
It will cost about the same, and perform better.
Either way you go though, should be a upgrade.