I've read the official guide about different densities and resolutions, but cannot find how to use different assets for the screens with the same density? For example, 320x480 and 480x800 use the same mdpi, but obviously it would be better not to scale graphics programatically and use pre-scaled assets. Is it possible to force application use different graphics for different resolutions with the same dpi?
Android version 2.2+, normal and large screens
The section of the Developers Guide you should see is Supporting Multiple Screens.
If you want to use different drawables for different-sized screens at the same dpi, then implement layouts and drawables in different size and/or display-width buckets.
The Guide tells you in what order to name the resource-path qualifiers. Or, you can let the XML file creation wizard in Eclipse do it for you.