I am not an expert...by any means. But will attempt to answer as best I can
The first # (DDR3xxxx) refers to the maximum clock speed the Ram can operate at (in this case, 1.866 Ghz.)
The second 3 (PCyyyy) refers the maximum transfer rate that the memory reaches, in MB/s
Clock speed is backwards compatible (Mobos can normally be throttled down in clock speed.) I don't think the data transfer limit matters....it should work, just at a bit slower transfer rate.
Best thing to do is check mobo documentation if you aren't sure.
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