if youre only getting one card and not CrossFiring it, then 500W PSU should be more than fine.
DDR5 is better than DDR3. i can tell you that.
also, dont be suprised to see older models are still out performing newer models.
theres a reason why those newer models are on the cheaper side.
having good GRAM is important for your computer and how much graphic performance it can handle at once.
if the 5670 @ 512mb GDDR5 out performs the 6670HD, then i honestly wouldnt be surprised... you should keep researching and get more knowledge of it