Are you trying planning on upgrading soon in the near future after this setup? If not, then I would try for a cheaper motherboard as you wouldn't really neeed a Z68 chipset. A simple H61 board should suffice.
Yea, if you're planning on upgrading soon, I'll go with a solid Z68 motherboard; one that has all the features that YOU need/want. The 2100 should perform very well in most games, but if the game is cpu dependant & utilize more than 2 cores, then you might as well jump into the 2400 or 2500K now.
The 560 and 6850 should net around the same frames and doesn't matter if it's an Intel build; it's just a matter of preference on Nvidia or AMD.