Selfish gene isn't really a theory. It's a different way of looking at the same principle of natural selection. As Dawkins put it, think of a two-dimensional drawing of a 3-dimensional cube. If you stare at it long enough, it will appear to flip back and forth between 2 different orientations. Same cube, different perspective. We know genes affect the fitness of an individual organism, so the two are inextricably linked. However, focusing on one as the unit of selection provides a different perspective than the other. Exploring and understanding both views leads to a fuller undersanding of selection.
Memetics, as I understand it, is not a way of explaining the holes in the selfish gene idea so much as a tentative logical extension of it.
The Extended Phenotype elaborates on this aspect of selfish genes.