First number is the generation. 2xx, 4xx, 5xx, 6xx, 7xx and so on.Oh, I just figured since it is a higher number that it was an actual hardware upgrade. My apologies.
Look at the second number to know where in a series it resides.
x10 (eg 210, 610) - business cards, not good for gaming at all.
x50 (eg 650) - starting to get ok for 3d rendering. Not the best for games, but ok for cad and video.
x60 - (660, 660ti, 560ti) minimum you'd want for gaming.
x70 - (470, 570, 670, 770) sweet spot for gaming price to performance
x80 - (280, 480, 680, 780) high end
x90 - (690) usually dual gpu, or super high end.
Titan is kind of an exception.
Generally the card will equal the one below it in the next series. 570 is about the same as a 660, 670 is about the same as the 760, 770 is about the same as 680. And so on.