What is key difference between different series of video card OF ATI AND NIVIDIA?

I need a graphic card for playing games in pc of series ATI Radeon x800 or Nvidia GeForce 6800

but confuse about their other series of cards too. for example i need a 256 MB card of Nvidia Ge Force 6800,but there are also other 256 MB cards in NVIDIAa other series.

pleaseexplainn me all the key difference for making different series in video card.|||There is very little difference between ati and nvidia. Just get the biggest number, eg. 9800|||This is a problematic question as some games favor one brand over the other. I have a NVidia (what most next-gen games favor) 9600GT and I can play Call Of Duty World At War 2 with No lag and with MAX settings. BUT some games (Very few) will not work at all with the video card because the designers made it more for ATI graphics cards. So really what kind of games wil you play? I prefer NVidia since more game developers make games for that than ATI (even though i like ATI too). I don't know if I answered the way you wanted but thats just my 2 cents.