The Atom is fine for Plex, just don't try to transcode any video, hah - it sucks at that. 99% of everything I watch doesn't need transcoding though. Sometimes the audio needs transcoding, but the Atom 230 in the FX160 handles that.
I have an old Core 2 Duo 2.66Ghz laptop that can handle 2 streams transcoding at once with Plex. You can buy a refurb Core 2 Duo for less than $200. Why bother with an Atom?