ive seen that stepping reach 2.6ghz easy and some have had 2.8ghz on about 1.5v on air. If you can get that stepping i would definately go for it. The CCBWE is the stepping and the 0551 means it was made on the 51st week of 2005.
I believe that they are, in the same way of the single core ones. I think it's just the FX's that get the honour of the fully unlocked multiplier, but they also get the price tag to go with it! 8)
im thinking an x2 might be best for me as im on phase too. The clocks from the opteron are very tempting but the risk of coldbug is higher than that of the x2.