I do some cuda programming. There are three big differences between the 5xx and 6xx architectures:
1) 5xx shaders run at 2x speed, but there are fewer of them...this is a good for some applications, bad for others, but the 6xx chips use less power
2) the 6xx chips have reduced double-precision arithmetic units. Games do not (for the most part) use doubles, so cutting this for gaming cards again reduces the power consumption and allows other parts to run faster
3) the 6xx has new compute capabilities (look up "cuda compute capability level") mostly around scheduling. This is definitely a good thing, but most software doesn't take advantage of it yet.
I don't know how much Blender depends on double precision, so that may or may not be an issue (if I had to guess, I'd say "not"). For future, the new compute capabilities may make a big difference...there are certainly some nice tricks I'd like to try out.
Sum: you will not get a 7x speed up, but you will probably 2-3x or more.
I'd get the 6-series, though it may take a while for drivers and other software to stabilize.