Random Access Memory Digital-to-Analog Converter. The part of the video card which converts digital intensity levels for RGB to analog voltages sent to monitor. A DAC can have RAM to store look-up-table (LUT) information - hence RAMDAC.
The speed of the RAMDAC (eg. 220, 250, 300, 350MHz) tells the maximum master clock rate, or video clock rate (VCLK) the image can be displayed at. You can calculate the maximum refresh rate for a resolution using a simple formula: VCLK/(height*width) = refresh rate