The argument over 256 versus 512 has been going for a while now. I'd like to throw a few extra facets out there, on both sides.
Firstly, now that the MBP uses DDR3 RAM at 1066MHz, there would be minimal difference between the data transfer speeds between the GPU's own RAM, and system RAM. Yes, there will be a large difference in latency, but if the texture files that the GPU are big enough, I doubt whether this would be an issue. Seeing as the 9600 has Turbocache, I'm sure this seriously reduces the benefits of large amounts of VRAM.
Yes, games will not benefit that much from more VRAM, because I imagine they involve rapidly swapping in and out different data about the virtual world as we proceed through the game. Therefore games would be bottlenecked by bandwidth, and how much texture and polygon information can be pumped to and from the VRAM, main system RAM, hard disk, CPU and GPU core. What about CAD programs though? These ultimately do not require rapidly changing texture maps, vertices and shaders, and therefore I would imagine would not require overly large bandwidths. Therefore, they are more likely to be limited by the amount of data about the object that you can fit in the VRAM.
Therefore, if you're a gamer, I don't think you need to worry about the VRAM too much. 256MB is enough for the moment. If you're into some seriously heavy CAD, maybe you want to go for the 512MB version.
