Android Market now allows you to upload screenshots in three sizes: 320x480, 480x800, or 480x854. Which size should I use?

The 320x480 screenshots look pretty blurry on my 480x800 Nexus One. But I'm afraid that if I use larger images, they will be illegible on 320x480 (my screens have a lot of text on them).

Looking at <http://developer.android.com/resources/dashboard/screens.html> it only shows densities, not screen sizes. Those densities are split equally between "Normal/HDPI" and "Normal/MDPI".

Would it be possible to upload both and have the market serve 320x480 images only to phones that need them?