I hate to use this term, but the "fragmentation" has started.
How will we handle questions that are related to this breed of phones?
low-end-devices as Vitor suggested might be a better approach because we shouldn't be grouping the different low end devices into different tags. In any case, we do need such kind of tag because this is a real problem some Windows Phone users are facing. Whether it's looking for an alternative, figuring out what app uses the most memory, wanting to know if their phone can run the next versions of Windows Phone or some other problems associated with such constraints; we should be there to answer them...
I don't know if we need to tag to 256MB or something else. Maybe we can use something less specific. Something like low-end-devices.