Good idea to have an index on datetime column?
Adding an index is definitely a good idea for this.  However, DATEPART(YEAR, datecolumn) = 2012 isnt' sargable so it will still do a scan of the index.
If you want it to use the index then you will need to do:
WHERE dateColumn &gt;= '1/1/2012' AND dateColumn &lt; '1/1/2013'

Please note the placement of the &gt;= and the &lt; signs to get the correct bounding box.