If this generation turns out as I'm expecting, Nvidia will need to release a bigger card for one reason, next generation games are going to be VERY gpu processing heavy and the 680 is very weak in that area. AMD have a balanced card, and all consoles have half the shader count of the AMD flag ship. So while this rumoured Titan might be large and hard to overclock, it could be the nvidia card this upcoming generation needs.

The hurdle for nvidia will be the fact that while the initial performance will be better than the jump from 480-580 a lot of gamers will feel it's still underpowered for its size and heat output. But to beat the 7970 hands down in those newly critical areas it'll need to be big unless nvidia have a real shader rebuild.