I have had experience with both the 256mb KO, and the 512mb golden sample 'goes like he ll edition' which I believe is slightly faster than the 512mb 7900GT you're looking at, but if that's anything to go by then your looking at a nice performance difference when playing at high resolutions, and when playing games with detailed textures such as oblivion.
However as said above the 256mb is just as good i'd say at 1280x1024 and below.