Can u substitute 333 mhz memory near 400 mhz memory ddr contained by a dell computer?

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Can u substitute 333 mhz memory near 400 mhz memory ddr contained by a dell computer?

No, do not mix bus speeds on RAM--you're asking for trouble.
Depends what computer you're using. Almost definately the answer is no. Computers cannot provide satisfactory power to run more power hungry memory than it was originally designed to use, and so you would ending up shorting the whole circuits. Very costly.
No, generally the higher speeds will drop down but not vice versa. If you entail memory i would go to www.buymemory.com