UIToolkit - Prime[31]: What screen resolutions do you build assets from?

So for Prime[31]'s UIToolkit, there is support for dynamic placement of assets based on phone screen resolution. I've come to understand that these assets cannot be scaled within UIToolkit (hence the purpose of HD textures) and that they simply appear to get "smaller" as the screen size grows.

My question is: what is the minimum screen sizes I should design for (SD and HD)? For example, if the minimum screen size I should build for is 640x480, then I would make my GUI fill to the sides and be something like 630 wide. That way, the assets continue to be 630 pixels on larger screens, even if other phones have a width of say 900.

What SD and HD resolutions should I design at? What's a good cutoff for SD to HD on Android? Thank you guys so much!