With the Steambox you can’t really tell if Valve will release it or not…..case in point, Half-Life 3 :p
In any case I would suggest you wait till June and make a decision after E3. That’s when PS4 and next Xbox will be fully revealed, and the only place Valve will have enough limelight to fully show off the Steambox, if indeed it’s coming this year.
If it is, then you can just do a quick specs comparison between the Steambox and what you can get from the market for that price.
Personally, I’d be happy with my existing setup (assuming it’s good enough to play Crysis 3 at High) and just buy a next-gen console. If, however, you’re current PC isn’t even good enough to play Bioshock Infinite, an UE3 engine based game, then yeah, wait till June and either build a gaming PC or buy Steambox - whichever gives better value for money.