The price increase for the HD 2900XT to the 1 gig version is do to the memory, bottom line. The price increase of the GTS to GTX to Ultra is do to the arch. differences. Also, as I said before, the varied resolutions don't show the whole picture of performance per game. Therefore, doesn't validate the performances of what the tested cards are capable of. There is no evidence that adding more memory allows for a faster video card.
Here is one example of what the
HD2900Xt can do at 2048x1536 using stalker (bottom of the page). This is just one example of what I found from many other reviews that show that the HD does get better at resolutions of 2048. However, this is not a resolution that most game at. This alone should tell you that if you are going to benchmark a game, you do so at every resolution. Not one resolution here, use another game, another resolution there, pick another game, etc.