You do realise that a DDR5 card is not necessary better than a DDR3 one,don't you?It mostly depends on the card's graphic engine,and if you take a look at some reviews(like this one http://www.neoseeker.com/Articles/Hardware/Reviews/amd_hd_6670_6570/ ,let's say)you'll see that hd6570 and hd5670 have more likely the same performance overall.
So,if I were you,I would go for the hd6570,given the fact that it has double the amount of memory and,on top of that,it's the newer card.