Nah, Seasonic builds very solid PSUs they're highly efficient especially yours since it's gold certified. And 650-Watt is plenty enough for a GTX-770, this card draws no more than 230-Watt of power when fully under load. So despite being a powerful card, the 770 doesn't draw that much power.
Anyways, I'd be greatly surprised that your FPS drops come from the PSU, I'd look elsewhere. Is that happening in only one game in particular, or every game? What resolution and settings are you using when playing? High resolution and 16x AA can bring even a 770 to its knees in poorly optimized games.