May I suggest this,PSU. Modular,so you only use the cables you need which means less clutter and better air flow.
Graphics cards comparisons.
HD6870 vs HD7770 the 6870 wins outright.
HD6870 vs HD6850 not such a difference.
HD6850 vs HD7770 the 7770 is still the slower card.
Unfortunately your first two links do not work so cannot compare.
I had before I upgraded to the GTX 670 a HD 6870 and a 500w PSU and I could play any game at high settings, so I can definitely recommend the 6870 and if you drop the 750W PSU down to a 600W PSU you will have a litle extra for your GPU.