For the games you said, yes the 6870 would max those games out.
For the PSU, with your budget and needing windows. No the CX430 is fine anyways. It's a solid PSU that'll give you headroom even with the 6870 and i5 3450 on load.
As for the 1333 vs 1600, there is very little difference. But B75 does support 1600 so if you wanted to you could go for it. You don't need it though.