Two devices for everyone
I think that sometime soon there will be a 100$ laptop suitable for everyone. And the processor and harddrive innovations will only benefit the server hardware. As with an internet connected cheap and reliable portable client hardware, you will have access to all multimedia files hosted on the internet.
Only profetionnals will need big desktop computers with fast processor and big harddrives. Normal people will suffice with light internet access clients. Though the set-top-box will be powerfull, in everyones living room, with even powerfull 3D console eigther external (like the Nintendo Revolution) or built-in (like the xbox360). That powerfull living room set-top-box will manage HD video playback and encoding for mpeg4-2 and h264.
So basically one portable pocket device, or 100$ laptop sized and one standalone home centric device, connected to a HD projector, that is all people will need. And each of those devices will cost 100-200$ (exluding harddrives). Thus everyone will have them.