No, it doesn't matter, mining uses no GPU memory. Go for the fastest card, not the one with the most memory.
That sounds like minors are inefficient then but i could be wrong. Why would manufactures add more memory if its not going to good use? My only thought is that its really for videogames to save non-visible data, like what's behind the player or texture memory or somthing but if thats the case it still sounds like minors can put this to some more efficient use no?
Well, you could argue that miners are efficient in their use of memory :-) Miners simply don't need much memory. You're quite right as to why manufactures provide the memory - the primary market for GPUs is for video, not OpenCL work. (While posting this, Luke-Jr posted about using the memory for swap, if that's possible - and I suspect it's not trivial - it would be very cool).
