980 Ti confirmed to have 6GB of VRAM and have a GM200-310 GPU. What the -310 means is anyone's guess, but the rumor has changed to the 980 Ti not being as powerful as the Titan X. While I wasn't hoping to hear that, it makes sense seeing as anything faster than the Titan X is likely going to cannibalize its sales.
http://videocardz.com/55419/nvidia-g...ure-6gb-memory
That's old information. Only 8GB is required and 16GB is recommended.
http://www.geforce.com/geforce-gtx-titan-x/buy-gpu
I cannot find the post at the moment, but Jacob from EVGA explained that if for some reason the VRAM needed to move into the system RAM, you would need that much available or else you'd likely crash. Seeing as most GPUs are only equipped with 4GB at most these days, this has never been an issue.