This page sums up the differences between
PS CS3 and
PS CS3 Extended:
Adobe - Photoshop CS3 editions
As I'm just a humble picture-snapper and occasional dabbler in 3D, I didn't think I needed the extra functionality of the Extended version, and I can't say that I've felt I'm missing out on anything. I would, however, definitely recommend upgrading from CS2 to CS3 if you can. There are a number of neat new features that come in useful for most
PS users (the
Quick Selection Tool as mentioned by shandy9 is just one example).