The colour depth a card can support depends on the amount of memory it has. VRAM has a second data port specially designed for the display refresh, making it much faster. It is becoming more common to find cards with 2M, and these will be able to support more colours, for example 1024x768 with 16 million colours. The actual resolutions and colours supported will, however, still depend on the drivers that are available. Some programs, such as Microsoft Windows, come with drivers for common graphics cards, but you may depend on the drivers supplied by the card manufacturers.
Many cards claim to be graphics accelerators.
These cards have an extra IC on them which does the calculations necessary to determine what parts of video memory need updating. This means it does not rely on the CPU and the amount of data transferred between the video card and computer is reduced, leading to faster screen redraw. Like all cards they must have the correct drivers, and you should check that the drivers work with the software you will be using. A number of alternatives to this have been proposed. While this supported 32bit data transfer, it still ran at only 8MHz, and like MCA is now being phased out.