I'd be curious to know if anyone succeeds on a 27". I did buy a 680M off ebay last year, but the machine would not boot with the card. No idea if it was a firmware problem, bad card, or incompatibility, since I don't have a PC laptop with a compatible slot.
My understanding is the 680M and 780M are very similar - maybe just slight frequency differences.
I actually read your thread during my research. It sounds like having a second monitor or windows would have probably allowed you to troubleshoot your black screen. More often than not I have only the second hdmi display working while the iMac display refuses to turn on. Selecting Mac OSX from Windows and shutting down is the only way to 100% guarantee the iMac internal display will turn on.
GMacUser, Thanks a lot for your work. I wanna attempt that on my 27' iMac with 780M.
Also, there's big different between 780 and 680?
In terms of upgrading your iMac, the only discernible difference between the 780M and 680M is that the 780M is twice the price. A 780m would be a great candidate for a EFI flash, however. If you managed to dump the ROM from a 2013 iMac with a 780M you could potentially get a fully supported bug free graphics upgrade.
However if you're looking at spending $850 on a top of the line card
today consider that you're gonna hit a wall with your CPU upgrade possibilities.... as nobody has been able to successfully upgrade to an Ivy Bridge with these iMacs.
If you do a lot of different types of gaming you're going to feel the limit of these locked Sandy bridge CPU's ( I already feel it with my i7 2600S while running dolphin,
except when setting affinity to force turbo boost)
In fact, most 680Ms on eBay are still priced around $650. Had I not found one for $360 I would not have proceeded with this upgrade. It was already seemed slightly irrational in terms of power per dollar considering it is a 2012 mobile card...
Look at it this way... the jump from my Radeon HD 6750m (512mb gram) and a 680M (4gb gram) in inexplicable. It's night and day, its a whole new world, its amazing. Super worth it in terms if the amount of games I can now play at MAX settings. I am confident I have greatly extended the gaming life of my iMac.
The jump from a 680M to a 780m is less noticeable... less worth it to be honest.
The 780M is a hell of a card, but if I was planning on spending that much money on a graphics card I would be worried about it being bottlenecked by future games. I would rather build a "hackintosh" with an i5 - 4670K ($230) with like two 770's ($850) or something. Its wayyy more bang for your buck.
Or honestly just sell your 2011 iMac add the graphics card budget and get the newer model. Just my two cents.
tl;dr If you have the money for that kind of (mobile) GPU don't waste it on a risky iMac upgrade, build a much more powerful pc/hackintosh instead.