But if you're not afraid of that, two 6770s should be faster than a 6870. As long as your motherboard is sporting two x16 PCI-E slots or two 8x PCI-E slots. If one of them is 4x, you won't realize nearly as much power.
This chart should help in showing you where cards fall in the order from best to last. The 6850 is only one level above the 5770 and the 6870 is only two levels above the 5770 so you would be better off getting another 5770 for only $100 and then later on when the new versions come out you can choose when to upgrade , either the 7000's or the 8000's , meanwhile you will be able to play games at a respectable fps and resolution. The forum rules want posters to list thier computer spec's so that in giving recomendations we know what you have and if it will work for you otherwise we are just shooting in the dark . You've told us that your psu is powerful enough but what brand is it and what wattage , how many 6 & 8 pin connectors do you have , what's your motherboard , how many x16 slots do you have? These are all things that would be helpful to know.
My psu is the CM gx 550w D3 model which is apparently really well made and got good reviews, not the E3 one which was made last year by seventeam (that one was bad). I was recommended something else by users here but I got a sick deal on it and took it.
I got a am3 biostar board that has 2 pci-e 8x 8x in crossfire and allowed me to OC my cpu (athlon x4) to 3.5* Ghz