I'd go for the extra 128MB VRAM before I would go for the extra processor speed from 1.5 to 1.67GHz.

BTW, Mac OS X's VRAM is allocated such that you don't make a hard split when multiple displays are active. If you need more memory on one and not on the other, that's fine. Only the memory allocated for the viewable display itself is allocated at all times. And at least a couple of backbuffers I suppose. But, 128MB is definitely preferrable to 64MB, especially when using shader programs, large textures, a lot of z buffers, etc. When 3D becomes trivial to use in Tiger you'll be seeing the graphics subsystem stressed a lot more often by many more applications besides games.

Bruno