goranpaa

While you do have a good chance of unlocking the cores of the X2 550BE, there is also a small chance that you wont. And even if you do unlock all the cores, it is not guaranteed that your system will be stable. If you really need a quad and you have the money? I say go with the 940.

Make sure you have a good cpu cooler slapped on when unlocking. The cpu temp will go up some.

This is the mobo on your list that can be used for unlocking the cores on: Gigabyte GA-MA790XT-UD4P + the F4G Bios. This, according to a compability list that I did find. The mobos, that are able to fix the core unlocking needs to have ACC support = "Advanced Clock Calibration Link" for AMD OverDrive (SB710 or SB750 southbridge).

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Case: Antec P180B + 2 X 120mm Noctua fans.
Psu: Corsair TX 750W
Mobo: Gigabyte P35-DS3 P.
Cpu: E6850 3.6GHz
Cooler: Scythe Ninja.
Video cards: 2 X Sapphire HD 4870.
W.D. Caviar 640 mb SATA 2 Black.
Sound: Auzentech X FI Prelude.
Speaks: Camebridge Desktop Theatre.
OS: XP Pro 32 + SP 3.