Fantatierchen: Well, with desktop grafics it is rather simple, at least now (higher number = better card, ti = enhanced version).
Nvidia notebook cards are a different case. Sometimes they just take old cards and give them new names, With a certain model they even did that twice, so that you had a card with the name of the current generation but with the physical properties of a card that was two generations old. This isn't only hard to understand that's fraud, if you ask me...
ATi sometimes does the same thing with their notebook cards too -_- and what's with the GT and GTX in nvidia names?