I believe the 4870 is the faster one. In addition its GDDR5 memory makes it faster as well apart from the architecture. However it is not the most efficient GPU there is. IMHO the HD 4870 runs pretty hot and loud.
Also believe DDR and GGDR are pretty much the same thing when it comes to video cards. Now the difference is GDDR5/GDDR3. Also kind of a waste of money to get a 2gb HD 6670 on a weak card.
Edit: Read thishttp://www.hardwaresecrets.com/article/DDR-vs-GDDR-Memories/168