I know plenty of guys running TWO 27" Cinema Displays off a SINGLE Radeon 5870 GPU with 1GB RAM, and they run perfectly fine.
Now let's jump TWO generations in GPUs. Either we're gonna get Radeon 78XX, 79XX, or GeForce 6XX cards. With regards Macbook Pros, we're definitely looking at the mobility versions. With the iMac, there's the possibility of either a mobile version OR a desktop GPU.
It' is ALMOST A CERTAINTY that ALL of these GPU's will have 1GB or GREATER RAM. Meaning the cards will be 2 generations above that of the 58XX in the Mac Pro, and have AT LEAST as much RAM.
If the 5870 can power TWO 2560 x 1440 screens, we need DOUBLE the GPU performance. What's the figures like for the 79XX and 680 cards? Are we looking at a 2X jump over 2 generations ago?
Don't forget, for gaming purposes, who's to say you can't jump back to 2560 x 1440 on the 27" or 1920 x 1080 on the 21"?
My greatest concern would be for the new Mac Pros... where there is the possibility Pros will use TWO of these enormous Retina displays... we'd definitely need TWO graphics cards, or something crazy like a 690 or 7990 dual GPU.
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I see no point in increasing the resolution of the iMac without increasing the screen size. The iMacs are already 'retina' at their respective view distances and an increased resolution won't give you any more work space (unless you want to read your text with a magnifying glass). The downsizes would be higher power consumption and production costs. Besides, there are no monitor panels in production which would satisfy the demand, and I highly doubt that Apple managed to build a secret retina display panel lab they don't have the technology to build panels like that given that experienced monitor makers only now start experimenting with those.
SHARP's been doing demos of an 8K display in relatively small 40 - 50" screens. Noting 8K is 7680 x 4320 pixels! So the density is likely in the realm of possibility, but I don't think it's possibly to do cheap for consumers this year.