It really depends on what your preferences are in terms of quality and performance. Here are a few questions to ask yourself along with comments:
(1) What resolution do you expect to play on?
This is one of the most important questions to answer when selecting a graphics card. The higher the resolution, the more horsepower and VRAM you'll need. For 1920x1080, any of these cards will suffice. 3GB is a safe bet at this resolution, as 99% of games don't even use 2GB of VRAM when maxed out (including antialiasing). Higher resolutions require more VRAM and 3GB becomes less desirable. 4GB or more is ideal if you plan on playing on 2560x1440 or go with a triple monitor display.
(2) What settings do you expect to play on?
A lot of people say "I want to max every game out!!!1" However, one does not truly max a game out if antialiasing isn't maxed as well. Some may disagree with that statement, but my point is that you need to figure out what matters to you. If you want to max every game out at 1920x1080 with antialiasing maxed as well, neither of these solutions will work in terms of purchasing a single card. 2560x1440 presents an even greater challenge, where you'll want to stick with a GTX 770 or above. The issue with the GTX 780 is that there is not an option greater than 3GB. While most games don't use over 3GB of VRAM at 2560x1440, the fact that some games do (such as Hitman Absolution) means that the future will present more of them. The question is when. It could be this year, considering the demanding games coming in the Fall.
(3) How much are you willing to spend?
I'm assuming you're willing to at least spend $650, which is the price of a single GTX 780. If you don't expect to spend much more and you don't see yourself throwing out another $650 in the near future for a second GTX 780, then getting multiple GTX 760s or GTX 770s might be what you're looking for. I'll break down the price points for you in terms of what will yield the best performance:
$650 max: 2x GTX 760 4GB
$650-1000: 3x GTX 760 4GB
$1,000-????: nx GTX 770 4GB, n=1, 2, 3, 4
The reason why I did not choose the GTX 780 is because your horsepower is essentially bottlenecked by your VRAM once you hit 3-4 GTX 780s. If a 4GB model was available, I'd replace the GTX 770 in my recommendation. Keep in mind, the GTX Titan offers 6GB of VRAM, which would be the Nvidia card of choice if you're looking to play on multiple monitors or just want the best possible setup (which would be 4x GTX Titans). The Titan is $1,000 though.
(4) Is Nvidia your only option in your mind?
AMD has some options that may interest you, depending on your price range. If you're willing to spend $1,000 or more, I'd recommend an HD 7990, which is currently the most powerful card on the market (excluding the Aries II). It's a dual GPU card, so you'll encounter the micro-stuttering issue that AMD's Crossfire presents, but this will be addressed eventually. Also, bare in mind that new AMD cards are around the corner (later this year potentially), so if you're willing to wait, their new series should outperform anything from the Nvidia 700 series. AMD also loves to bundle games with their cards, so you'll get some free games out of the deal.
I would recommend looking into the HD 7970 GHz, but it's a tad weaker than the GTX 770 and is a little more expensive, so it's no longer a recommendation I can make.
If you can answer these questions, I'll be able to provide deeper recommendations.
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