Question for my friends who use Macs

I have a 3.5-year old Powerbook G4 that's running Leopard. I have iPhoto 4, which came with the computer. I use it pretty extensively and have a few thousand photos in there. I export photos to Flickr pretty easily, and would like to be able to export them to Facebook (the plug-in for that requires at least iPhoto 5).

Is upgrading iPhoto worth it? Is there something else I should look at instead? And is there anything I have to watch out for with an upgrade? (I will check Google also, but it never hurts to get opinions from trustworthy sources)