Well in short when I got my 720 BE I was hoping for an overclock of 4Ghz but all I could get is 3.879 GHz.
But I will admit I was surprised when I unlocked it to a quad, that the thing could do 3.716 GHz.
I had my fun with it now it's degrading at first I needed 1.48 v for 3.716 unlocked then when I did it again I needed 1.52v to make it stable.
So now it sits in it's clam shell waiting for a new home or for me to get a LN2/DICE pot...lol.
Another point to remember you are going to be limited as to future proofing, the new Bulldozer will require an AM3+ Mobo.
So if I where buying with the end goal being the Bulldozer then i would get one of the cheaper, but still good overclocker of a board so it doesn't seem of such a stretch of a purchase to buy a new board when AM3+ comes out.
But even then the AM3+ boards are going to be able to house the older AM3 CPU's, so maybe hang about on a cheap substitute board until then, then slap in your AM3 Cpu into the AM3+ board and then you will still be able to upgrade to Bulldozer when you feel the need to.
But I think that even the thubans (x6's) are going to be fine for a while yet.