Yes a G-550 is plenty for a GTX 780..Plenty! - especially on a Haswell i 7 CPU.
(Call me crazy...)
I run a EVGA Superclocked GTX 760 on the small Seasonic G-360. (needed a 2x molex to 1xPCI-E 8 adaptor)
System:
i7-2600 (*Lowpower OC to 4.2 GHz w slight lowered Vcore) 8GB DDR3-1600, and just one 1TB Samsung 240EVO SSD. (No HDD)
Typical high gaming/graphics load is measured 220-250W on the PSU input (=max 225W on the PSU output)
Rock stable so far. Furmark and 3dMARK11 (Extreme) testet.
30Amps on one 12V rail on that little thing