You need to not worry about streaming porcessing units, and memory clock, and all that hoo-hah. Cards from different manufacterers (Nvidia and AMD) and even sometimes in different lines from the same manufacture dont follow the same performance for their "stats".
Think of CPUs, a CPU from 3 years ago that is 3GHz isn't going to perform the same as one that is new that is 3Ghz, there is lot more to it then what meets the eye.
With that said you just need to pay attention to benchmarks.
So you want to compare the 6970 and the 5850?
The 6970 is 22 percent more powerful than the 5850.
The 5870 is MORE powerful than the 6870.