There's no simple, one-shot approach, is there?
Well, yeah, there is really. Seth's approach - which is the same one I use, and the same one a lot of commercial games use - is just that. A one size fits all, if you will.
Clearly it's not possible to do the same thing in 2d. But the magic of 3d is that everything scales automatically, and with a little clever thought you can do almost anything in your game with complete resolution independence.
I suppose you could argue that you might want higher res textures if you're going to support 1024x768, but it's certainly not essential. And even if you do, it's quite easy to have a config option and switch between two sets of textures. I agree that this compromises your download size, of course ( If you're selling that way ) but I find download size is much more dependent upon music than textures anyway.
Ultimately, very, very few commercial games don't support 640x480. Those few that don't support it invariably have a *damn* good reason. The Sims, for example, where you really need the detail. Very few games need it.