You'll need to decide on 3 factors, that are linked:
- resolution: the higher resolution you want, the higher the price of the card needed
- quality: same as above: higher anti aliasing will require a bigger card
Typically, you can sort graphic cards into multiple categories, based on performance and price:
- ultra high end ($700+): Runs your game at max settings on 2500x1600+ resolution (GTX590/Radeon 6990).
- high end (~$400): runs your game at max setting at 1920x1200+ (GTX580/570/R6970/6950)
- middle high: (~250) run max settings at 1650x1050 (GTX 560Ti/R6850)
- middle: (~$150) run max settings at 1280x1024 (GTS550/R6770)
- low end (<$100) not really for gaming
So, decide on your resolution and your budget. Remember you can increase resolution if you decrease game settings (for example, you can run a GTX 560Ti @ 1920x1200, but at lower quality).
one guide of current great graphic cards and budget (amongst others):