For Nvidia, anything ending in -30 or less is a cause for the manufacturer to have a laugh at you giving them your money for a piece of junk. A card ending in -40 is acceptable if you're on a tight budget and can do without good performance. A card ending in -50 offers good price/performance value, and a card ending in -60 actually starts getting decently fast. A card ending in -70 is high performing, and a card ending in -80 is the top of the line product.
The 100's-digit in front of the is the generation. A 7xx series card is newer than a 6xx series card which is in turn newer than a 5xx series card and so on.
Sometimes, cards with a similar numbering scheme have extra words, such as "Boost" or "Ti", tacked onto it. In that case, the one with more extra words is generally (not always, but generally) faster.
The 100's-digit in front of the is the generation. A 7xx series card is newer than a 6xx series card which is in turn newer than a 5xx series card and so on.
Sometimes, cards with a similar numbering scheme have extra words, such as "Boost" or "Ti", tacked onto it. In that case, the one with more extra words is generally (not always, but generally) faster.
erformance ratio for Nvidia cards at the moment. I'm personally not familiar with the ATI lineup, so someone else will need to chime in for their equivalent price-point alternatives.