If it's under AppleCare, you will get an exact replacement. And I will also hazard that the 680m probably wouldn't work as a replacement for 6970m in your iMac.
Yeah I know they do a like for like but if it was the same work then I'd pay the difference. I guess the drivers are in the software but I'm not sure how interchaneable are the.gpu's
Have Apple replace the GPU under AppleCare, or ask them nicely whether you can get a replacement machine? If they say no, then let Apple repair your machine, sell it after .. then use the money toward the new 2012 iMac. Why risk the whole machine for something you don't know will work? Clean and simple, no?
No, its not about price difference but the internal layout is different. You will never have a 680 as a replacement for any pre late 2012 imac.
Agreed. Plus GTX 6 GPUs on the new iMac comes in BGA chip, not as MXM daughterboard just in Radeon a.k.a soldered on board. So it's not just about whether you're willing to pay the difference to Apple, but it's also impossible and not available.
Well, there is a way to upgrade your graphic card, using a Thunderbolt-to-PCIe enclosure, which is currently windows-only.