Within one major version of NVidia (and ATI) cards and sometimes two major versions, the higher the last two numbers are is what tells you the ballpark performance rating.

Examples:
NVidia 480 > 470 > 460
NVidia 460 > 550
NVidia 470 > 560
ATI 5870 > 6950
etc.

Generally, these charts hold true: http://www.tomshardware.com/charts/2...ndex,2673.html

There are caveats people will snipe but that's generally a good gauge of where they rank.