Hi I'd like to keep the price range under 300 if possible so 7950 max. It's a Cool master 550w extreme power plus silent 12cm fan. I wonder if there will be a bottle neck for current cards. Also I am on 4 Gb ram (ddr3 1600)
Yes, a second 6870 would be a great choice at 1080p, except that you'd definitely need a new PSU.
If $300 is about your max and you definitely want a new card, I'd say either stretch to get that 7950 I mentioned, or go down to a 7850 or 570. But tbh, at 1080p, I would probably stick with the 6870 a little while longer. It's a great card, and you might want to hold off so you can see what the 660 Ti is like (and what price changes it might bring).