Some people here don't know much about PCs
RAM on video cards is simply for storing things like textures and vertex data.
For games, more RAM (above what is required) does not make them go faster. The problem comes in when you don't have enough RAM. This means that things have to be put in slower storage.
For example, textures go into system RAM. And if you have not enough normal RAM, parts of your game may even end up in virtual memory - the hard disk!
I hope this helps explain some things...