To explain AMD's numbering of their cards -
The first number is the generation of the GPU. So 7XXX would be the 7th Generation of Cards, 6XXX would be the 6th Generation of Cards etc etc.
The second number is usually related to the card's architecture.
The third and fourth is the model of the card.
Higher numbers are not necessarily better, but for most cards the rule applies.
AS for the best card/price value, I'd reccommend the GTX 660Ti. It's cheap but uses the same architecture and GPU chip as the GTX 680 (GK104), and if you overclock it then you'd be able to reach decent speeds for any game.