Is this the motherboard you're looking at
? Seems ok, though it doesn't have support for USB 3.0 or SATA 6.0 Gb/s. Neither of which are necessary, but it's something to consider.
All of the parts look fine.
1) I'd make most of my decision on a graphics card based on price/warranty, I don't think brand matters a whole lot.
2) The new 6850 and 6870 have similar performance to the 1 GB 460. The 6850 is slightly slower, the 6870 is slightly faster.
3) Whichever is the least expensive.