The 6870 is the best performer out of the three and the cheapest, nothing else matters, get the 6870.
The one that costs $439 is a gimick with overclocks and non-reference cooling.
The one that cost $300.00 is a reference card and that's the suggested retail price from AMD/ATI
The one that costs $270 is newer tech and costs less to produce.
As you can see on this chart, the 6870 is in the same calss as the 5850 but in reality it edges it out in overall performance.http://www.tomshardware.com/reviews/gaming-graphics-radeon-hd-6870-radeon-hd-6850,2782-7.html