WHAT'S VRAM?  (5.1)
-------------------
       Video RAM is where the computer stores the images displayed on 
  your screen.  On some earlier Macs with built-in video (Mac 128, 
  IIci) this was kept in main memory.  However it's considerably more 
  efficient and faster to store the screen image in its own separate 
  RAM.  Generally the more VRAM you have the more colors or shades of 
  gray you can display and the larger the monitors you can use.  The 
  chart below shows the number of colors that can be displayed at a 
  given resolution with the specified amount of VRAM.  Monitor size 
  has no direct relation to the amount of VRAM required though larger 
  monitors normally support higher resolutions.  Larger monitors 
  just have fewer dots per inch than smaller monitors with the same 
  resolution.  Also note that simply because a particular video card 
  or Mac has sufficient VRAM to support a given number of colors 
  doesn't mean that it actually can though more modern cards and 
  monitors typically do support several resolutions.

Resolution   512x342 640x480 832x624  1024x768 1152x870 1280x1024
VRAM 
256K               256         16         16
512K             32768      256         256          16             16
768K          32768      32768        256         256            16            16
1024K        16MIL        16MIL     32768        256           256           16
2048K        16MIL        16MIL      16MIL      32768        32768        256