Usually in video cards, the higher the number, the better the card, for instance 5700, 6600.
The next bit your looking for is the Direct X version it allows. Of course, all new cards support Direct X 9. Nvidia supporting Direct-X9c, the newest form of it.
Finally, there is Vanilla cards (the standard, for instance 6600), then sometimes depending on make, PRO models, then GT in some, and finally Ultra being the best, so this is always worth while checking out.
I for one don't look too much at memory speeds either, more towards pipe lines as well, but this is just a proof of point, the higher the number, the more pipes it probably has, and also the GT, Ultra thing too
The pipes is what powers the rendering of games by the way, adn the more pipes, the more complex things the card can do in a pass (for example, texture, bitmap, etc)
I hope this helps