It's so pointless now, both companies have so many cards out now the performance difference is extremely minimal to warrant a whole new card. It could easily be reached with a mild OC. There's little point of the X800GT when there are X700Pro's and X800SE's and X800 Vanilla's to fill the gap (though I must admit it's a great deal, though the X800 is still almost the same price). Then the X800XT's, from XT to PE the differnce is all in the OC. Then the X850Pro's are similar to the X800XT's and the X850XT is similar to the X800XT PE and so on.
NVidia is just as bad, they have 6600's, 6600LE's, 6600GT's, 6800XT's now, 6800LE's, 6800's, 6800GT's, and so on, do they really need all those? Me thinks no.
I guess the marketing ploy is the make each card out to be blisteringly fast so the customer gets confused and just ends up buying the more expensive model as they assume it's better. Seems to be working though.