I might be a little late on this, but:
say, for an x550 graphics card.
The middle digit is more important than the first digit.
You can't buy a card in the 00-30 range, and 90 will probably cost you more than half a thousand dollars.
A 570 card will get you more than 740. Though I am a bit out of date on which generation is up *shrug
A few more examples:
A 4-digit graphic card model number is just a 3-digit model number with a 0 on the end. It follows the same rules/meaning otherwise.
Model numbers cannot be compared between AMD and nVidia directly, always refer to benchmarks.
680 always better than 580 (generation +1) of the same manufacturer (AMD or nVidia)
680 always better than 670 (model +10) of the same manufacturer
The last digits being 90 or higher often means a dual-gpu card, which are often not worth the cost and can't use both gpus in all games. Each gpu on its own is often of x60 or x70 grade.
The last number not being a 0 typically means it's a revised card which may or may not be significantly better than the xx0 version. Do your research with these. e.g. the geforce 275 was actually more powerful than the 280.
OC / pre-overclocked etc versions can be anything from 1% to 10% better than the standard version, which may make them more powerful than the next model up, or may not. Again, research here.
More graphics ram typically means bugger all.