The most important part is the graphics processor on the card. Doesn't matter what other bits you have, if the graphics chip is cruddy, the card is cruddy!

Next most important thing is the speed and responsiveness of the RAM chips. The colockspeed of the video RAM is measured in Megahertz, and the responsiveness in nanoseconds. More MHz is better, less ns is better.

The amount of video RAM included is a distant LAST in consideration of how good a card is. Given the same graphics processor (and processor clockspeed of course) , a card with 128Mb of fast RAM will outperform a card with 256Mb of slower RAM. And the 256Mb versions often include slower RAM!

There's also the "width" of the "data highway" to the RAM to take into consideration. For example, two cards with 600Mhz DDR RAM. One uses a 128-bit memory interface, the other a 256-bit memory interface. The 256-bit model has twice the transfer rate!

Most high end cards have a 256-bit memory interface (keep an eye out for SE, EZ, Lite editions though - these are usually crippled versions).