I think many of you are far overestimating the cost of producing 5k iMac displays. Samsung currently has one on sale for only $599, at 28". That's only about $300 more than a high quality 1080p display. Source:
http://www.amazon.com/Samsung-28-In..._sim_pc_4?ie=UTF8&refRID=0FJ6PZFPRS03SC9Q6C2Z
Not only that, but iMacs currently carry an enormous profit premium, which I believe is around 50% on the high-end models. It's not unimaginable that Apple could release a retina iMac with only a small price increase, and simply eat some of the rest of the cost as the price of keeping us with the competition.
As for GPU's, Nvidia's 900 series of mobile GPU's are really quite remarkable. If Apple does go with AMD, I hope they're using an unannounced new product, or perhaps even a desktop-class GPU. Personally, if I were going to buy a 5k display, I'd want some serious graphics power behind it. Lack of graphics performance is why I'm using a windows desktop right now instead of an iMac. If anyone at Apple is listening, two GTX 980M's in SLI would be a great way to go. Either that, or give me a full desktop card.