1) No 4k monitor can be driven at full resolution and 60Hz right now, even in OSX 10.10 Yosemite (true or false?)
false. I have the monitor Samsung 28D590D and it runs 4K at 60Hz on OS X. I am using Yosemite but I think it would work on Mavericks as well. I did not do anything special, no boot flags.
2) Full resolution can only be achieved through display port (true or false?)
True. It is a data bus limitation. HDMI 1.4 cannot give 4K at 60Hz. When monitors with HDMI 2.0 starts to pop up, that will probably change.
3) so which card would be the cheapest one to drive a 4K monitor?
Well, I had the same question myself. I went for a GTX 770 just to be sure. It seems to be more than enough. I can even do Dota 2 gaming at 4K!
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