Manufacturers often quote megahertz (MHz) numbers. There are usually three clock speeds quoted: the core clock speed, the memory clock speed and the Ramdac clock speed. Core clock speed refers to the internal speed of the graphics processor. Memory speed (or memory clock) refers to the speed of data transfer between the card's onboard memory and the graphics processor. A clock speed of around 500MHz or greater is best for 3D graphics and games.
Ramdac speed refers to the capabilities of the digital-to-analogue converter that provides the graphics output from the card. It doesn't directly affect graphics performance but governs the range of refresh rates the card can support at a particular screen resolution. Ensure that the card you buy can display the resolutions you want to use with at least a 75Hz monitor refresh rate to avoid eye strain from screen flicker.