Another thing worth noting is that most of these benchmarks done at really high resolutions with AA enabled are most likely making the 512Mb cards run out of memory, which will lower average framerates quite a bit. If you look at benchmarks that list the 8800GTX and Ultra along with the 512Mb cards, the GTX and Ultra tend to perform much closer to the 280.
The 280 is 32% faster than the 9800GTX and only 11% faster than the 8800GTX at 1920x1200 in STALKER.
Just something I noticed.
BTW, has anyone seen any Mass Effect benchmarks? It runs fine for me at 1600x1200 with no AA, but another 20fps average and 4xAA wouldn't hurt.