What do they do?
These components, either built into the motherboard of your Mac or on a separate internal graphics or video card, build the image to be displayed on your monitor, and convert it into the signal sent down the video output lead to your monitor. Among many quite complex tasks, they have to perform any colour conversion required (possibly using look-up tables), maintain a bitmapped copy of what’s being displayed on the screen, and ensure that the output signal is correctly synchronised to refresh the screen 50 or more times every second. How do they work?
Specialist chipsets have been developed, and so that the same chips can drive a wide range of different monitors at different resolutions and screen or colour depths. To perform these tasks efficiently, the number of colours which your Mac can display is determined by the amount of video memory (VRAM): 256 colours at 640 x 480 pixels need 0.5Mb VRAM, while millions of colours at 1152 x 870 need 4Mb. On some more basic models, video memory is borrowed from main memory, but on most it’s completely separate. What can go wrong with them?
Problems normally only occur if you’re trying to drive an incompatible monitor or are using the wrong lead. The chipset works out the abilities of the monitor by autosensing it very early during the startup process. Incorrect connections can confuse this and may even damage the monitor. If you only buy and use monitors and cables known to be compatible, you shouldn’t experience any such problems. How can I upgrade them?
Some models with limited amounts of video memory can be upgraded with more memory, to allow you to use more colours at higher resolution (if your monitor supports those resolutions). The modules to be added are different from main memory, and must be specified carefully for use as VRAM for that model. If you have a free expansion slot, you may be able to add a more capable graphics card (possibly with an accelerator), and Macs can then support two or more monitors even spreading windows across them. Further info
Web sites with further information include: Apple’s complete database of all Macs at support.info.apple.com/info.apple.com/applespec/applespec.taf; the Guide to RAM Upgrades (including VRAM) at www.newertech.com/software/guru.html, and EveryMac, with comprehensive details of video chipsets and memory, at www.everymac.com | 
