As the link I provided above shows Crossfire HD 7770s need a recommended 520w power supply. The Seasonic is a very high quality unit that will have no problems.
More proof if you need it.http://www.guru3d.com/articles_pages/amd_radeon_hd_7750_and_7770_review,7.html
" Our test system is based on a power hungry Core i7 965 / X58 system. This setup is overclocked to 3.75 GHz. Next to that we have energy saving functions disabled for this motherboard and processor (to ensure consistent benchmark results). On average we are using roughly 50 to 100 Watts more than a standard PC due to higher CPU clock settings, water-cooling, additional cold cathode lights etc."
Measured power consumption R7770
1.System in IDLE = 155W
2.System Wattage with GPU in FULL Stress = 231W
3.Difference (GPU load) = 76W
4.Add average IDLE wattage ~10W
5.Subjective obtained GPU power consumption = ~ 86 Watts
So add another 86 watts to the 231 watts and you get 317 watts max load with 2 x HD 7770s. And that is on their system using 50-100 watts more than your will! HD 7770 is a very low power card.