I share the dilemma of the OP. For me, I'm sure the high-end 21.5" would be enough - FOR NOW.
I'm not a serious gamer, and don't particularly care about the high FPS shooters that seem to dominate discussions about what GPU/iMac you need. I would, however, like to be able to play games like Skyrim, Dishonored, and Rome TW/etc. - AND their descendants that come out over the next few years - at decent settings. Other than that, I need a computer for everyday academic work.
So it's really a question of what games like this will be like in a couple of years - and what specs they're likely to require. It's clear that the 27" iMac with 680MX would be the safest choice, overall. But I wonder if that's overkill. And like the OP, I'd really rather not have to get the 27": it's just so much BIGGER than necessary; I'd greatly prefer the 21.5 (I mean, I'm sure it's great for games, but for everyday use, I just don't think I need that much real estate; and as I've said elsewhere, having that screen staring down at you is intimidating!
). If there were a better GPU for the 21.5, there'd be no question....
What do people think? It seems to be the hardcore gaming crowd that's insisting the 680MX 27" is absolutely necessary to game; does anyone have any thoughts about what sort of future-proofing is needed if I'm mostly interested in keeping up with RPGs and strategy games for the next few years?
MagicThief, thanks for starting the thread. We seem to be in the same boat, and have similar concerns. Have you reached any conclusions?
Thanks!