Basically, forget the product numbers altogether, it's just marketing schmoo. Sometimes, they cover up thinks like older chips branded into new product line as lower and card to sell the surplus.
Basically, you have to watch for the following parameters: used chip (and subsequently architecture, for example the Kepler chips brought huge performance gain), VRAM amount (larger is better for detailed textures), core clock speed, VRAM speed, bus width, number and type of shaders, etc.
Read on each item and learn how do they affect performance in specific scenarios (for example, older games won't utilize so many shaders and texture processing units, while newer games will).
