seems like help is rare now a days.
I'm as lost as you on Nvidia rivals...espeically since it seems like they all have A LOT lower stream processing units than ATI.
Looks like the 6870 just has higher stream processors, ~100mhz higher core clock. As far as real world performance, they are probabaly not far off from each other.
What games do you wan to play? If you google the name of hte game the card, you will usually pull up how many FPS you get on what settings.
This is the difference I pulled up from newegg:
comparison link
screw them both... get a 6950x2