Easiest way is to use the Outlook Import wizard if this is a one-time event
(if it's going to be a repetitive event - an alternative solution may be
more suitable - ContactGenie comes to mind as one of the choices :-)  - link
below)

Excel vs CSV - basically a matter of choice in this case -

1) If Excel - you need to create a proper "named range" covering the column
header row and all the data you want to import.
or
2) If you don't want to mess with "named ranges" - export the worksheet as a
.CSV file and import it directly into Outlook.

Either of the above will do exactly what you want (this assumes that your
installation of Outlook has the necessary import filters installed or if
not, you have the install disk when it asks for it). Didn't ask for Outlook
version since the process is pretty much the same for all versions.

Karl