If you had your photos organized by folder beforehand you can import those folders of photo into iPhoto and they will end up as rolls in the main library window. The roll title will be the same as the title of the folder you imported. That will give you your first crack at organization. As QuickTimeKirk said, do not ever move, rename or do anything inside the iPhoto Library folder while in the Finder. All organization should take place in the iPhoto application's window.

Besides albums you can create smart albums based on some criteria you set. It can be a date range, a keyword, a file name or part of a file name, etc. You can also create folders in the Source pane to hold albums, slideshows, books and other folders. Those folders cannot hold photos, however.

And if you af the patience, assigning keyword can be very convenient later.