yes socket A is older.
754 is for the slower Athlon 64 cpu's, however thy are or will be making the slower Athlon 64's for socket 939 as well.
940 is mostly for opteron, and servers, cause 939 requires ECC-ram... error corection controle or something. its slower and more expensive ram.
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u can see cpu type for the socket, just go to
www.newegg.com , go to the cpu section, and search acording to interface type.