How important is the order of columns in indexes?

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I’ve heard that you should put columns that will be the most selective at the beginning of the index declaration. Example:

CREATE NONCLUSTERED INDEX MyINDX on Table1
(
   MostSelective,
   SecondMost,
   Least
)

First off, is what I’m saying correct? If so, am i likely to see large differences in performance by rearranging the order of the columns in my index or is it more of a “nice to do” practice?

The reason I’m asking is because after putting a query through the DTA it recommended that I create an index that had almost all of the same columns in it as an existing index, just in a different order. I was considering just adding the missing columns to the existing index and calling it good. Thoughts?

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