My rule of thumb: if you are not sure whether you need Photoshop or not, you probably don't.
I would recommend trying The Gimp first, you will finds tons of resources to help you out (I particularly like the video tutorials at
meetthegimp.org). Then, if ever come the day that you say "OK, I've mastered this thing and I need something more powerful", or "I need specifically this functionality from Photoshop and it is impossible to do it with the Gimp" it might be the day to spend big money on Photoshop CS3.