I don't know where to start. Your right 256 bit is faster but it's not a huge limitation, and the memory bus width really has nothing to do with the amount of VRAM. It's like saying someone with only DDR RAM wont benefit from upgrading his 1GB of RAM because he doesn't have DDR2 or DDR3. It's a limitation in speed but has nothing to do not needing or using more ram. I can't explain it anymore clearly. If you can actually find an article or benchmark to prove what your recalling from "memory" I'd love to see it.
Quad G5 hits the nail on the head perfectly: "I believe the one advantage of having more video memory is that the texture swapping into and out of the frame buffer will be less, and the advantage is better when using a higher resolution screen."
I didn't say the bus limits the amount of VRAM, only the efficiency in which it's addressed (i.e. speed, logically).
This is the topic, and post, in question:
https://forums.macrumors.com/posts/6997797/
Here is a comparison of the 256 and 512 model of the 9600M GT:
http://www.macworld.com/article/136251/2008/10/macbookgraphics.html?t=204
There doesn't seem to be any reasonable difference between the 256 and 512 model according to these tests. However, what Quad G5 said does indeed sound logical, I agree. But only in high resolutions presumably. However, wouldn't the same be true for, say, a 1GB 9600M GT, in that it outperforms the 512 model, since more data can be loaded into the VRAM without too much swapping. Couldn't the GPU, by that logic, load even more data into the VRAM when even more memory is used (for example 2GB, or 4GB), and hence benefit from it, which would result in performance boosts? Certainly, there must be a point where speed (128 bit, not considering the DDR type which is the speed of the RAM itself) limits the efficiency of the VRAM. Maybe it's not precisely at 256 MB, but it can't be much higher I think.
The difference between VRAM and the regular RAM (in your example) is the speed in which it needs to be addressed, I believe. Your GPU needs fast memory. One would benefit from upgrading to 1GB DDR RAM because speed isn't the bottleneck for regular RAM. You don't need that kind of speed for regular computing. If speed wasn't an issue, than why would a GPU need its own VRAM?
Anyway, I don't really know much about these things, but intriguing nonetheless
