You do not have to take the 70-640 and 70-642 before taking the 70-646, you can take them in whatever order you like. However, you must complete all three before you have earned the certification. It is highly recommended that you have the knowledge and experience required to pass the 70-640 and 70-642 exams before attempting the 646 exam because there may be some carry over, but it is not required.
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CompTIA A+, Network+, Server+, Security+
MCTS:Windows 7
MCTS:Hyper-V
MCTS:System Center Virtual Machine Manager
MCSE:Security 2003
MCITP:Enterprise Administrator