I didn't know the info below. then everything is fine and dandy. It would take a lot of ram if they kept 1080p 60FPS (partially) unencoded in the memory.
If we have 6GB GDDR3 for games alones I'll be a more then happy Batman.
What will happen is that the frame would be read from the front buffer, sent to what ever hardware encoder they have and then sent to RAM.
They will not be storing lots of lossless frames.
BTW
I doubt you would be happy with 6GB of GDDR3.