A tip before you start:
To select many pictures at once in the Finder, type Command + A to select all pictures. Now hold down the Command key and click on any pictures you want to deselect.

Export pictures

Start by selecting a picture in Photo Booth and typing Command + R. This will open the folder containing the pictures in the Finder. Select the pictures you want to export and drag and drop them into an album in iPhoto (assuming you’re using iPhoto to manage your photos).

Delete pictures

Begin with the same procedure as with exporting by clicking on a picture and typing Command + R. Select any pictures you don’t want and type Command + Delete. Refer to the above tip for how to easily select many pictures at once.

Update (09/19/2007):
Someone has created an AppleScript to automatically import Photo Booth pictures into iPhoto.