In the current lineup of nVidia cards, the first number is the generation of the card, the second number is it's relative performance within that generation. For the most part a generation jump will not overcome a lower 2nd number, i.e. a 670 is not better than a 580 in performance, though it is better in efficiency. Oddly enough, even 2 or 3 generations are sometimes not enough to overcome a smaller performance number. i.e. a 650 does not perform as well as my GTX 460, and if the generational performance increase continues as has been happening, the 750 when released still won't perform as well as my 460
Breaks down like this for nVidia cards:
x10, x20, x30, x40 = Budget / extremely low - low performance cards (Not gaming cards unless you're rockin 1024x768 resolution or lower)
x50 = entry level mid performance
x60 = mainstream performance
x70 = upper level high performance
x80 = premium performance
x90 = Ultimate Performance
And the only letters you want to see after the number is Ti i.e. GTX-660 = mainstream performance, while GTX-660 Ti = enhanced mainstream performance. All "Ti" really means is that the card doesn't have as much stuff dissabled as its non-Ti counterpart.
Best card in the $250 price range Radeon 7870 like this one: http://www.newegg.com/Product/Product.a ... 6814102983
Better performance than a GTX-570 & at lower power consumption, heck it's even better than the GTX-580 in certain games & setups.
Radeon's are on a similar system but with an extra number... 1st number = generation, 2nd number = performance segment, 3rd number = relative performance within a given segment.