I'll admit a lot of this stuff goes over my head - So perhaps I'm missing something here, but surely the graph in the first post (and other benchmarks) shows that although some games use over 768MB of memory, it doesn't make *that* much difference to gameplay?
Rroff mentioned Modern Warfare 2, which was missing from the graph, but
this benchmark shows that the 1GB version of the card still only gets just under 5FPS average more than the 768MB card. (looking at the standard versions of the cards, as different manufacturers overclock in different ways)
With Crysis Warhead, which I imagine also would take a lot of VRAM, the difference is again only about 5FPS more.
With the original Crysis,
this benchmark shows that at the highest settings they tested, the average FPS is only 1 higher with the bigger card. Clearly neither card is good enough for that game at those settings - But at lower settings the difference is still only 1 or 2FPS.
Certainly in the future a lot more games will use larger amounts of video memory, but by then the rest of the card will be so outdated that it's not going to matter that much.
As I said, a lot of this stuff goes over my head, but such a tiny increase in FPS doesn't seem to justify the extra £40/£50 for the bigger card.