Is the new Date better than the old Date?

We have a CalendarTimeUTC Dimension table in our Data Warehouse that looks like this:

The PK in the table is the CalendarTimeUTCId (clustered). This used to be a Int field. All fact tables have CalendarId in them (some of these are multi-billion row partitioned tables).

We want to go from putting a DATE value to DATE+HOUR value in this field.

Sample data (Old and New):

Now with the advent of SQL 2008, and the shiny new implementation of DATETIME, is there a reason to switch the dimension's ID column from INT to DATETIME?

How will it affect the Index Size in the fact tables? More importantly, how will it affect performance?

Datetime datatype takes up 8 bytes. Int datatype takes only 4 bytes. If you wanted to convert to a date datatype (because you needed to make use of date manipulation functions, for example), I'd suggest using smalldatetime which only takes 4 bytes.

As far as indexes and performance: because the indexes would be on the same physical size of data, I don't believe you'd see a performance hit nor an increase in the size of the indexes.