About Shared Memory

The MacBook uses the Intel GMA950 graphics controller for video output to the built-in display as well as to the mini-DVI port. This graphics chip set is different from Apple's other portable products since the graphics chip shares the main system memory of the computer rather than having dedicated graphics memory. The GMA950 allocates a base amount of 80 MB for video and boot processes at startup. In a MacBook configured with 512 MB of SDRAM, this leaves 432 MB of SDRAM available to Mac OS X.
Mac OS X may make additional main memory available to the graphics processor for texture use beyond the base 80 MB amount mentioned above, depending on the application being used. The most common types of applications that request more system memory to be used as graphics memory are 3D and graphics-intense applications.

Graphics Performance

For best graphics performance, your MacBook ships from Apple with two equal-sized SO-DIMMs installed, one in each slot. When memory is installed in your MacBook in equal-sized pairs, the computer gains some performance benefits from the memory being interleaved. If you upgrade the memory in your MacBook, make sure you have matching size SO-DIMMs (both in terms of memory size and in speed) in each slot for best graphics performance.