IMO, 512 MB on any current card is stupid, by the time games actually require 512 mb (and not D3, with it's uncompressed ultra textures that only waste memory), the gpu itself will be too slow to run them with all the eye candy that would take up 512 mb.
128mb gf4200 = stupid
256mb 9800 pro = stupid
512mb x800xl = ? (hint: rhymes with stupid)
Of course, if they can sell these cards, and they do, then only the ppl who buy them are stupid, and the manufacturers are smart enough to profit from the consumer's stupidity.