You got the wrong CPUs. You needed to get Opteron 200 series. Also, you won't need the heatsink since it already comes with two.
Your motherboard is a Thunder K8SR S2881
240, 240EE, 242, 244, 246, 246HE, 248, 248HE, 250, 252, 254, 256 (1.4 - 3.0 GHz)
260HE, 265, 265HE, 270, 270HE...