Why go for a 6870 when the newer 7850 beats it by a decent margin in pretty much all games, overclocks better and only cost an extra $10.
As for the PSU, something around the 450/500w would be more than enough. But you really need to look for a good brand. PSU's are a very overlooked component, a lot of people think a 500w PSU of eBay for $10 is the same as a 500w $80 PSU from a brand like Seasonic, they are both 500w so they're the same, right?..... WRONG!
Cheap PSU's are inefficient, very unreliable and ain't properly tested to make sure they do what they should do. A cheap PSU might start pumping too much juice into a component like your CPU or graphics card and before you know it, you have a dead PC! Or it will just be so inefficient your system will be unstable as it's not putting out enough juice so your system crashes or freaks out.
Seasonic are regarded as the best of the best, but they come at a price. Antec, Corsair, XFX, Be Quiet, OCZ are a few decent brands that are a bit cheaper. In that order of quality.