The HD 5670 is not powerful enough to take advantage of 1GB of RAM regardless if it is DDR3 or DDR5. Based on benchmarks, at most a HD 5670 1GB card will give you 1 extra frame per second.
The basic differences between the HD 5770 and HD 6770 are:
1. All HD 6xxx cards are only PCI-e 2.1 card. No PCI-e 2.0 cards are produced.
2. The HD6xxx cards have HDMI 1.4a connections as opposed to 1.3c. Generally speaking it can handle 3D movies and 3D games.
If 5670 is same tier as 250 (which i think they are ) the ddr5 will get you a couple extra frames per sec , but i'd have got a 450GTS DDr5 if it would have fitted easily in my Dell case.