To clarify, I was wondering about the intended use of this CPU (Gaming , home theater PC,......etc). But if you're going to buy the x2 550,you're better served with the 555 BE.
I would recommend buying Athlon II x3 or Athlon II x4 since the L3 cache isn't worth sacrificing one or two extra cores. I recommend staying away from the Core2Duo as it'll never be able to outperform Athlon II x4 in the same price range. The cheapest Core 2 Duo on Newegg is the E7500 & it costs $118. I think the intended use of these processors would only be as upgrades to squeeze longevity out of an aging 775 system but not for building a new system.
Athlon II x4 630: http://www.newegg.com/Product/Product.aspx?Item=N82E16819103704
Athlon II x3 440: http://www.newegg.com/Product/Product.aspx?Item=N82E16819103843