Memory clock is how fast the Graphics cards memory is. On the 7750 the DDR5 version is much faster than the DDR3, the 6670 is a less powerful card so it won't make as much of a difference, but it'll still be noticable.
...it's how fast it can move information to and from the memory. Memory bus width and memory speeds determine the bandwidth. The higher bandwidth, along with other factors, the better fps, scores, you get.