I think the only way to accomplish the different levels of magic would be to have a totally different set of classes for each of the levels of magic. A wizard in a high magic setting is going to have access to a lot of spells. You put that same wizard in a low magic setting and you either have to take away most of their spells, which nerfs the class in comparison to others, or you have to face the wizards throwing off the whole "low magic" thing.
There was a book called "Swashbuckling Adventures" by AEG, which was the d20 version of their Seventh Seas campaign setting. It was quite well done. However, if you mix the classes from that world, which are balanced for low magic, with the classes from the PHB, which are balanced for high magic, then it gets really ugly. I know this from experience as we made that mistake in one campaign.
I think, however, that a well designed core system would allow for the core campaign setting and then others which are at a much different magic setting, and still allow the game to be played reasonably well. Right now, we do not have that. Too many high level encounters are based around the idea that everyone has oodles of magic and magic items. This dependence means that classes designed for a low magic world might find themselves unable to compete when facing high level challenges. I am hoping that 4e is actually going to move away from the current magic toy dependence, which hopefully change the dynamic enough to make the choice of setting more flexible. I do still think that you'd need to make new classes, especially spellcasters for low magic and possibly fighters for really high magic.