To calculate how much video RAM you need, multiply the width times height of your desktop area (in pixels). Take that product and multiply by the bit depth (8 for 256 colors, 16 for high color, 24 for true color). Now divide by 8. This is the total number of bytes of video RAM you need. Divide by 1024 to get the number in kilobytes.
You may download a handy utility to swap from one resolution to the other. This utility also will allow you to adjust your refresh rate. This tool is graciously provided by EnTech Taiwan. There are also related tools available from their website at http://www.entechtaiwan.com.