If I elaborate it a bit:
In GTX, the name for new GPUs is a tri-number. Here the first number describes the generation (or the series to be precise), the 2nd one the level of power and the 3rd one is usually a 0.
For eg: A GTX 670.
Here 6 is the series, 7 is the power level (high end) and 0 is a 0 lol.
Frankly speaking, the first and last number hardly matters, if we take a GTX 660 and 750, then the 660 will be better.
In Radeon HD series, the name is quad number (Nvidia had such names too, like 9500). The first number is series, second is the level of power, so is the third one, but the third number matters less than the second number, the fourth is 0 again.
For eg: HD 7770.
Here 7 depicts that this one is from 7000 series, second 7 says the level of GPU (mid range), the third 7 says the level of power (medium again) and fourth is 0.
Simply put, a 7850 will be better than 7770, because, as I explained, the 2nd number is the deciding factor and other ones are not that imp (however the third number is somewhat imp).