No such thing as "overkill" when it comes to performance, there's only "over-budget". A 780 may seem excessive for a 60Hz 1080p screen, but for a 120Hz, it's still not always enough, so it's a matter of matching your expectations to the capabilities of your hardware. Do you only have a 60Hz screen? If so, then yes, you don't really need more than one 780. There are only a handful of games that a 780 cannot provide playable performance in when using highest IQ settings and high levels of anti-aliasing, so you will be just fine.