For multiple clients (aka multiple graphics surfaces) memory will widen the "window" of optimal performance. For most normal folks, the higher core count will give you better per-pixel detail, which is nice for single clients. For us, multiple clients sucks up a lot more VRAM, especially if you're using ISBoxer to downsize higher resolutions.

What I don't know is where the threshold is for the different games. That will depend on your detail settings and which game it is, etc.