New to Outlook 2007 is the Desktop Alert that will display a minature window showing you that new email has arrived.  You can control how long the window displays as well as the transparency of the Desktop Alert.

1.  Click on Tools, then Options.
2.  On the Preference tab, click E-mail Options.  
3.  Click Advanced E-mail Options.
4.  Click Desktop Alert Settings.
5.  Click and drag to shorten or lengthen the length of time a message is displayed or how
     transparent the message should be.  You can preview the option to see if you like this setting.
6.  Click OK until you are back to the main Outlook window.