It all comes down to what you'll be playing, if it's a TV or a Monitor and the refresh rate of that display.
Gaming in 4k is the equivalent of trying to play 1080p across 4 displays. 4k will require a load of gpu cores and a lot of video memory, plain and simple. Another thing to consider is that SLI'ing isn't a 1:1 performance increase, you're usually only getting about 75% out of each card because they're trying to work together to render a continuous picture. Also, it doesn't double your video memory count - it will only see the video memory count of a single card. That's why they started building cards with SLI'd GPUs on a single card to try and compensate for the inability to pool video memory across 2 cards.
Your power supply should be fine unless you have a crap load of drives, a massive cooling element and dual Titan cards. So, a pair of 780ti's will get you the GPU power you would need to push 4k but 3GB of video memory will be very limiting for games with lots of texture effects and mod layers like Skyrim. I initially hoped Nvidia would release a 780 ti with 6GB of memory since they release a 780 with double the ram but I kinda doubt they're going to release that this late in the cycle since they're working on Maxwell. So, if you wanted to be able to play at a high frame rate on 4k res on a game with lots of added effects options, you're stuck looking at a Titan or TitanZ. I think an individual TitanZ is OP enough to destroy 4k gaming regardless of the title for at least the next 2-3 years.
Next, the display - there are a lot of different monitors starting to release that support 3840X2160 but the refresh rate and connection requirements seem to vary from display to display. In some cases, you'll need 2 separate display port connections from the GPU to your monitor. Ideally, you want one of the ones with a 60hz refresh rate. Most of the 4k TVs on the market are going to support 60 Hz, 120 Hz, 240Hz or 480Hz - depends on the quality. Pretty sure most of those will function via a single HDMI cable, but I haven't tried connecting one yet so I will have to check.