For NVidia, for +/- 1 generation of cards you can usually ignore the first number unless you're really after that extra 5-8% performance bump. (i.e.: 460~=560, etc.). The x60/x70 cards are the faster clocks/wider memory buses and such. The lower numbers (x20/x30/x40/x50) are the "value" models of the same GPU class.

Not always precisely the same scale but you get the idea.