Photoshop CS3 Extended has a bunch of stuff in that's largely specialist purpose and irrelevant to mainstream digital photography. There are a few features that can be useful - but you probably won't miss them (saying that, I have Photoshop CS3 Extended, because it's part of Creative Suite 3 Design Premium). Photoshop CS3 is the version Adobe recommend for photographers.
However, if you need to integrate 3D images, work with DICOM files (medical imaging) or need some of the other specialist features in Photoshop CS3 Extended, your decision is made!
Think seriously about buying Creative Suite 3 if you're going to spend the money. Creative Suite 3 Design Premium is around US$1800 - for that, you get Photoshop CS3 Extended (US$1000 on its own), Illustrator CS3, InDesign CS3, Dreamweaver CS3, Flash CS3 Professional and Acrobat 8 Professional. If Photoshop CS3 will do (the non-Extended version), and you don't want the web stuff (Dreamweaver and Flash), look at Creative Suite 3 Design Standard at around US$1200. Creative Suite 3 Design Standard is a much more rounded offering than Creative Suite 2 Standard as it includes Acrobat, which the corresponding CS2 product omitted.
To give an idea of what a good deal you're getting in the US compared to the UK (yes - I know it's another thread, and Adobe have had a lot of negative publicity about this), at current exchange rates, I paid as much for a Creative Suite 3 Design Premium upgrade from Creative Suite 2 Premium as Creative Suite 3 Design Standard costs in the US - though I am including the 17.5% sales tax.
Looking at it another way up, Creative Suite 3 Design Standard is Photoshop (around $650) and Acrobat (around $450 - most people can find a use for Acrobat). For the extra $100 for the suite, you get Illustrator, InDesign, some fonts and a few other bits and pieces. Design Premium is the cost of around three applications - and you get six.
David