I haven't experienced 4k gaming myself except with the DSR factors setting in the NVidia control panel and I will say three gtx 680 4GB cards can't do it, FPS are too low.
If you want to future proof to an extent the titan x would be the way to go for 4k. There are some games now that use 6GB of vram at 4k and new games coming in the future will only use more. I'm with Korth, 2 titan x's minimum for 4k gaming to ensure 60FPS. Crysis 3 even drags 2 titan x's to its knees at 4k with FPS averaging in the 40's.
Now let's talk about monitors. I been looking for days at monitors and right now the biggest 4k monitor with g-sync is the acer 28" but is still a little small for gaming to me and has a TN panel. A TN panel has the fastest response times down to 1ms which is what we gamers desire but TN panels don't have the picture quality or color accuracy as an ips/pls display.
So the question is do you want a monitor with g-sync with a fast response time and slightly lower picture quality or do you want a bigger monitor with a better picture. I will sacrifice g-sync in a heartbeat for a bigger and better picture as long as the response time is reasonable.
The fastest non g-sync monitor I've come across is the Benq BL3201PH. It's a 32" 4k ips monitor with a response time of 4ms. And the price is better than most others out there at 979.99
http://www.ncixus.com/products/?sku=101284