The 256mg actually slows the cards down a little as there are more chips and they have to run them a little slower.
Right now there is little to no need for the 256mg versions of the 9800 and 5950, it is mostly marketing.
Best bet, get a 128mg card or wait for the next gen. of cards to come out that can put the extra memory to better use.
Oh and I have a overclocked 9800 128mg pulling in 6K in 3dMark 2003