it really depends on how someone classifies a generation then

4800 to 5800, die shrink and larger chip thus doubled
5800 to 6800 was just optimization and so minor gains, even though its a year after
6800 to 7800 should be about double due to the major shrink happening, but thats easily another year away.

so overall its about 50% perf increase per year
perf/$ however im not so sure about. when the 5800s came out, perf/$ didnt drop a dime, for months, and in the last 2 years things have only become a little better looking at the overall picture. im thinking of getting a 6950, but its only ~3x faster than my 4850, but the current price of the 6950 is also about 3x that of the old price i paid for my 4850 about 18 months ago

in summary, i dont think perf increases are really happening as fast as people think.