Any Atom without an ION is basically something that can't do much graphically. In Windows I've had them struggle at times to do basic DVD level video.
You don't want a D500 series Atom because that is a desktop processor and will needlessly waste battery life. The N500 series Atoms + ION are the lowest you should go if you want HD playback to have a chance.
As for the AMD side the C-50 is slow for CPU but will easily handle HD playback. The E-350 is the best of "netbook" chips overall if you can find it in a netbook under 12 inches.
However, I find that Nvidia's Pure Video HD decoder and the APIs/programs that use it such as DxVA and VDPAU look the best. That's just my personal experience but it only comes with a bit of tinkering so I doubt it will matter that much to the majority of people.
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