Hi.
I'm thinking about BMPC as well.
I've 2010 MP 12Core 2.66 with 24GB RAM, ATI 5870 and NVIDIA G120.
I was making same researches and find out that I need some GTX card - month ago or so EVGA announced 2GB GTX 680. It is quite pricey ($600) but it is flashed so if you need boot screen for windows purposes there it is.
If no, you can take GTX 580 2.5GB - Probably fastest mac card for Resolve these days.
You can also buy refurbished flashed cards for mac at ebay.
I have Nvidia G120 for GUI.
I probably will need more RAM. Now with 2x 3d applications and Photoshop it's almost enough, so I will need to buy more when I will start to shoot 4k.
My CPUs will probably work fine for this kind of workflow.
I have no clue about 4k monitors. I have 30' 2560x1600 Eizo for my own but I'm thinking about 4k because of downscaling it to 1080p I won't bother with that for now.
iMac is nice option cause it have 680mx as a choice in fastest model that will work great for resolve purposes.
But iMac is not option for me, I need swappable disks.
I hope you will figure out something.

Greetings
Tom