Frenetic Pony
Veteran
We can already see the GTX 970 struggling in some games with its 3.5GB pool of fast memory, and the R290X 8GB is sometimes significantly faster than the 4GB variant. So it's not unlikely that games would need more than 6GB within a couple of years, especially in usual circumstances (4K, supersampling, VR, stereoscopy, multi-monitor setups, or any combination of the above).
GM200 has a 384-bit bus, so barring a hybrid memory configuration, it was either 6GB or 12GB (3GB is clearly not enough).
As if framebuffers take up nearly that much memory, and the 970 struggles because it's memory is divided into full bus bandwidth and slow bus. The game using it inevitably doesn't know there's 2 memory pools, so it's up to the drivers to keep resources out of the 512mb of slow memory if possible. When that fails the bandwidth access times spike and framerate drops.
You simply won't need 12gb. It's questionable how much you'd need 8, with about 5gb of ram reserved for apps on the consoles you'd need less than a gig extra even to run a 5k framebuffer. Hell making even a crazy 256bit per pixel framebuffer at 5k gives just a 480mb framebuffer. And 8 full 32bit render targets stored at once is supremely high. Good luck getting a 384bit bus to supersample that. No, just like last time the new Titan is clearly dedicated to compute, and at the PCB size and sheer cost of the ram will no doubt have a cost to suit.
The upcoming Nvidia and AMD launches look like a repeat of the Titan/780ti v 290x launches. Similar performance goals (probably +/-15% of each other) with Nvidia clearly aiming at high profit margin compute customers with a huge expensive card with a lot of ram and AMD clearly aiming at lower profit margin/higher volume gamers with a lower price and less ram (4-8gb?).