smlg.ca:
Welcome to the Apple Discussions. I suggest you go to Apple's iPhoto
site and look at some of the tutorials there on using iPhoto. It will show you how to organize your photos with iPhoto.
This shows how you can use folders and albums within the left hand Source page to organize your photos, slideshows, books, calendars, etc.
Picassa is primarily an image browser whereas iPhoto is an image management application. Lots of differences there as far as organizing files by keywords, etc.
Warning: If you decide not to use iPhoto but have imported your photos into iPhoto and then deleted the original files, do not follow Lego Mania Podc..'s advice. That will delete the remaining copies of the full sized files on your HD. (Then are or will be, if you use iPhoto, contained in the folder titled Originals that is inside your Pictures/iPhoto Library folder.) However, do not ever go into the iPhoto LIbrary folder via the Finder and move or edit any folder or image file. All that has to be done from within iPhoto.
I do believe if you give it a try you'll like it. Why not first start by importing maybe 10 of your folders into iPhoto and then experiment with albums, folders (in the Source pane), keywords, etc.
Do you Twango?