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Yeah not retina. It's amazing. The 775m 2gb and the 780m 4gb are really good cards and the i5's are solid, mines the 3.4 ghz version with 2gb Gtx 775m and a fusion drive. Lots are listed at 1400 odd but you can pick them up for 1k which is decent IMO. I paid 1.1 and had it delivered to my door by a decent chap. The 775 is on par with the m290x I believe but as its non retina the gpu isn't as taxed. Played borderlands at 1440 last night maxed out and it was smooth as hell. Also glad I didn't split the fusion drive as I have 70gb of apps (that might include games but not sure). I can't be happier to be honest, although my impatience resulted in me missing out on some better auction deals.
Also http://blog.gameagent.com/wwdc-2015-why-metal-for-mac-os-x-is-a-big-deal-for-mac-gaming/?utm_source=GameAgent+News&utm_campaign=de6f23e8b7-Weekly_Roundup_133&utm_medium=email&utm_term=0_51457f1248-de6f23e8b7-49881669
Yeah, I don't really know much about all that but I remember asking the same question as you did and finding an answer or "has to be written with/in metal" or something to that effect. Yep, bootcamp only for a long time. I miss the infected too atm.
Couple of solid days on this new iMac, work and play, it's mental good. Ordered some more ram to really max it out.