A 384bit memory bus, with 3GB of VRAM. In Crysis 3 I'm using more than 2GB of VRAM on my 7970, at 1080p. So definitely will want to look into getting a card with a beefy memory setup.
This may mean going for an AMD card, they are the ones that really dominate when it comes to memory and high resolution gaming.
I know amd offers higher memory interface than nvidia, but I do plan to SLI and from what I've seen SLI has far less issues than CF.
I'm not going to get a card under 3gb of vram, but I'm not sure if that will be enough for the next gen games coming out in the next year or 2. I'm just curious how much memory others are using at this high of resolution on games like crysis 3.