Stop thinking about a fixed list of screen aspect ratios and just make your layout so that it looks nice in any reasonable screen dimension. A good UI for pretty much any task should be able to stretch to fit any dimension nicely. If you use a lot of images, make them nine-patch where possible. Things that just can't be stretched non-uniformly can be scaled to fill or fit.
The different DPI options are there just so that you can provide higher-resolution images for higher-resolution screens, they're not meant to solve the screen dimension differences.