The basics regarding the Nvidia naming convention is as follows. The number series (200, 300,400,500) is the generation of the card, with the newest being the 500 series. Within each series, the last 2 digits denote the relative performance of the card within that series. So, a 570 is a more powerful, higher model than a 560. So, not all 500 series cards are better than 400 series cards, or even 300 or 200 series cards. For example, I have a GTX 285 which was near top of the Nvidia line a few years ago. According to the performance rankings I have seen, it performs about as well as new GTX 560. However, it still beats the crap out of a GT 520, which is a current generation, low-end card.
By the way, about 5 years ago, Nvidea changed their numbering system, going from 4-digit numbers to 3-digit numbers; from 9800 to 100. So a FX 5800 (4-digit number) is a very old model card, while a GT 545 (3-digit number) is a current generation model.