That bit width is nothing to do with the ram, it is the bus connection between the ram and the gpu and it does not make it run faster.
It does allow more data at once to be transferred at the same time and allows more data per second overall given the ram is the same in both cases.
It may or may not make a difference depending on game and settings but it is not given that a graphics card with a higher bus width will be faster.
You cannot make any fast and hard rules which has really been the case since ati have supported memory types other than gddr3.
Look up reviews for either the card you are looking for or any equivalent before purchasing, do not look at specs.