I was going to write a long "paper" about memory controllers, number of ram chips, bits transferred per clock cycle, etc, then I realized you probably wouldn't understand any of it. The easiest way to explain this is that the memory bus in bits relates to the memory bandwidth for the card. And the more bandwidth the card has the faster the GPU can get info. The best way to compare cards is to look at the benchmarks between them. Realize that the Quadro will not make a good gaming card.
If you really want to learn about this go search online about memory bandwidth. Come back and ask any questions you might have.
Great article. A little dated, but the fundamentals are the same.
It all pretty much has to do with bandwidth and potential memory throughput.
The higher the number the more bandwidth; the newer (gddr5) the more bandwidth as well. It doesn't matter so much which is higher or newer, what matters is the benchies and which card is faster because each manufacture can compensate for slower ram speed/ lower bandwidth...