[Dahak]

I've a MacBook Pro 15" with the maximum RAM (4GB) available at the time of purchase. It has 512 MB of VRAM. I was told that paying for the extra 512 MB of RAM is not worth it due to the VRAM. I was quite uncertain as this doesn't sound right

Forgive me if this vague as my memory is spotty, but the arguement was along the lines of the fact that the OS would never access the remaining 512 MB of RAM because either A. The RAM was needed to support the VRAM or B. The VRAM would be used by OS in place of the RAM and that the OS couldn't access more than 4 GB.

Is either argument true?

[Big Mac]

Yeah, most of that information is wrong. RAM and VRAM are completely separate on the MacBook Pros and have nothing to do with one another. The only thing that is true is that 4GB is the maximum amount of RAM addressable by that model.

[tooki]

VRAM is video RAM, used by the graphics card for 3D rendering, mostly. For everyday use, the amount of VRAM makes little difference, but for things like 3D games, CAD/CAM, and professional media applications that have been deliberately written to use the graphics card for acceleration, it does help.

As Big Mac said, the system can't use VRAM as extra RAM.

[SierraDragon]

In addition to graphics apps that often do benefit from the additional VRAM, OS 10.6 "Snow Leopard" due in the next few months will move graphics card usage into mainstream apps via the OS. Full interaction of OS 10.6, graphics card power and apps will no doubt take more months, but IMO your extra graphics power is not "wasted."

From Apple's website:
Another powerful Snow Leopard technology, OpenCL (Open Computing Language), makes it possible for developers to efficiently tap the vast gigaflops of computing power currently locked up in the graphics processing unit (GPU). With GPUs approaching processing speeds of a trillion operations per second, they’re capable of considerably more than just drawing pictures. OpenCL takes that power and redirects it for general-purpose computing.

-Allen Wicks

[Simon]

OpenCL will offload some processing to the GPU. But nobody can tell you right now if that will also increase VRAM demands beyond current minima. Many operations that are very fast on GPUs do not require large amounts of VRAM. They require bandwidth.