How is 6670 behind 5850?
Isn't that a newer model?
In ATI model parlance, the last three numbers are supposed to correspond with the target market from generation to generation. 3850 > 4850 > 5850, etc. They actually changed it in the 6 series so 6950 is the successor to the 5850 (the 6850 is actually slightly slower than a 5850, though not by a lot).
Anyway point being that a 5850 has about 2.75x the computational power of a 6670 (this according to ATI's own published specs). 6670 is a mid-market card at best. I would say more like low-mid market. The 5850 was one step below top of the line when it came out, and is still vastly superior to any number of "current" cheaper cards.