Here's what they look like in order from most to least powerful.
Those prices for those cards aren't really a "deal".Those are normal retail prices for any of these cards.The most bang for your buck I would say get the 6870.The 6870 is as fast if not faster than the GTX560 but for $20 less and for brand new.
Sapphire 6870 $170=$150 after MIR + Free Shippinghttp://www.newegg.com/Product/Product.aspx?Item=N82E16814102948
What are your full system specs?(CPU,RAM,etc.)
What is the make/model of your PSU?