I was reading the specs of ati radeon 5770 ddr5, and I noticed it's 128 bit (Please correct me if I am wrong). Does it mean it has no added benefit compared to cards that have 256 bit but are ddr3? (I am only asking you to consider ddr3/ddr5 aspect)
This is a trade between bus size and memory clock
in reality they are not directly comparable by memory type/bus, for solid compassion you should check out the reviews of this GPU.
There are many other factors that impact the speed of a GPU such as memory latency and on GPU cache that they dont advertize that are just as important.