If you're somewhat pc literate, why would you buy a steam box? I would wait at least until it is out for half a year to see if games are actually optimized for it and if it's a good system.
Otherwise, building a gaming system right now is easy, just build a decently powerful pc and throw in a 150$ graphic card and you can play 80% of the games at high settings@60fps and the rest still at much better quality and higher fps than the consoles.
Heavy duty work pc and a tablet with a keyboard seem like the best bang for the buck too me, unless you need a laptop for work on the road.
Of course, if Valve prices it's box very competitivly it could be interesting, but closed system and pc are just two things that don't go together for me.