PSA - prototype steambox with a GTX 780 is powered by...

[thedosbox]

For those who think they need big power supplies:

http://www.ifixit.com/Teardown/Steam+Machine+Teardown/20473

That's a i5-4570 + GTX 780 being powered by a 450W SFX PSU.

Insert usual caveats about 12V capacity, PSU quality, number of drives etc etc etc.