May I ask what model number motherboard you are using? IIrc the 6400 chips are AM2 socket CPUs, but they also work in AM2+ boards. Similarly the Athlon II X2 250 is an AM3 socket CPU that also works in many AM2+ boards. Therefore if you have a good am2+ board which offers newer bios updates then you may be in business.
That said, if your issue with the cpu was that thermal paste poured into the CPU socket then the board may also be damaged.