I'll do my best to comment on these:

1. The main selling point of Extended is indeed its 3D capabilities, especially the ability to paint directly on 3D models. Frankly it's not a patch on Maxon's BodyPaint and if you really need to do that then a copy of BP is a better buy.

2. No, I don't think so. The impression I get from the Adobe site is that if you buy the collection you can't upgrade individual components of it. The Adobe store says 'Products within a Suite cannot be upgraded individually.'

3. I didn't have any issues running PS CS4 and Illustrator CS3 on Vista and they're fine on win7. One point to note (which I certainly didn't realise, though I should have done) is that if you're running Vista or win7 64 bit, both 32 and 64 bit versions of the software are provided, BUT most (read virtually all) of your plugins are still 32 bit only, so you may be restricted to the 32-bit version even on a 64 bit OS.

4. Well.. the main one is probably how many versions can I leave it before I upgrade again, the cost being what it is. I tend to skip alternate versions but even that's questionable, especially with Illustrator which doesn't change that much between versions.

Steve