The assumption around these parts seems to be that Season 5 of GoT will combine material from AFFC and ADWD, and I think that's reasonable: I certainly can't imagine that cutting out Dany, Jon and Tyrion from season 5 would go down at all well with show-watchers.
Furthermore (and I say this as a big fan of FFC and DwD), it's also assumed that the relatively "talky" nature of these two books, which are short on action, will mean they're unlikely to be able to be stretched to more than about 1.5 seasons: again, this is something I'd agree with. I see season 4 having perhaps 10% of the FFC/DwD material, season 5 covering the bulk of it, with season 6 finishing up the remaining 15-20% or so before moving into WoW territory.
My concern, though, is this. Compressing the two very large novels into about 1.5 seasons means that Season 5 will introduce a hell of a lot of important new characters, whom TV viewers will have very little time to bond with. Off the top of my head, I can think of at least:
At least one Sand Snake
The Kindly Man
Lady Stoneheart (who I'm counting here as a new character)
As being relatively important new characters, important on the level of the introduction of Stannis and Melisandre in Season 2. On top of that, there's a whole number of others who, while perhaps "cuttable", will likely be named secondary characters, such as:
Another Sand Snake or two
At least two of Arianne's co-conspirators
The High Septon
Homeless Harry Strickland
And I've probably forgotten many more.
So, the question is: how does the TV show both include such a veritable flood of new characters without forgetting half the old ones and losing all sense of pace?