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Refer to the MCEM post on the site...it should give you enough info for the same.
For starters,
only preclinical subjects will be asked...in the MCEM part A exam,
but there can be implied topics that are clinical in nature. for eg. they had asked the A-a gap calculation in the jan 08 mcem part A, which is actually not basic science but is still a part of it.
Read the text books that you are comfortable with, dont mug stuff up, because the exam has all implied questions only.
My personal suggestion... Kaplan for USMLE part 1, with special emphasis on pharmac, anatomy and physiology. So read textbooks for atleast these subjects.
also atleast give a single reading to basic sciences for MRCP by Phillipa.J.Easterbrook, its a great resource. Also has a question bank in the form of a supplementary book which is useful.
refer to online sites like mcem.org which have practice questions.