If you're a casual photographer, there's nothing about Photoshop CS5 (a.k.a. version 12) that requires you to buy it. Photoshop 5.5 of the 1990s has everything you need today, so long as it runs on your computer.
CS5 is all about getting more done faster, which in this iteration, is all about better, faster selection tools and automated lens correction.
Photoshop CS5 12.0 Open Folder.