Brilliant. This is what I have been dreaming about for years!
PCs are not sci-fi, they ARE indeed like trucks. What you say and what I dream is where PCs should be heading toward.
One small tablet device to go but it's "2"... whenever you need the full horse power, you just attach it to a monitor or make itself bigger and you play your games on it.
That being said, I think we are heading in that direction already but almost... with RemoteFX, maybe stuff like http://thinlinx.com become a little tablet in your hand where it can use the power from the server and just stream the stuff. Kind'a like "OnLive" http://www.onlive.com/ ...and this way, we will soon have both a small good to go tablet and the full PC as long as you got a good line ofcourse.
Although we can really move into the domain of upgradable mobile deiveces. I remember there is something called VidDock. I find it really cool. If we really made a full fledged interface to connect graphics card externally, we are essentially able to game Crysis at High on a notebook. I wasn't able to try it out because it is using notebook card, which is already phased out on my HP notebook.
Imagine an interface that allows you to add additional CPU, RAM, GPU using a simple unified external connection, I think we would be able to phase out desktop at most cases. Because wouldn't it be cool to have everything on your notebook and when you are at home, you can upgrade your notebook for higher processing load. No need more data sync, no need or two copies of software, no need for two desktop, and much much more.