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The main two manufacturers are INTEL and AMD.
The 4th quoter results of 2008:
INTEL - 80.96%
AMD - 10.6%
AMD is the ONLY compeditor of Intel
AMD
amd
You can use an amd sempron or an amd athlon II with an am3 socket, but when am3 was released it was meant to be used with the amd phenom line of CPUs.
You can use all three: amd sempron, amd athlon II & amd phenom.
amd is the best