Once you have your Google Apps for email running you may choose to continue using your MS Outlook client to synchronize to your Gmail account via imap.
Here is a step by step guide to do this in MS Outlook 2007;
https://mail.google.com/support/bin/answer.py?answer=77689
Most clients do however prefer to completely do away with MS Outlook and use the Google service, this is of course a personal choice.
Please note, the MS Outlook synchronization may take several minutes to update depending on your internet connection.
