The key difference between those is that the 5870 is quite a bit faster than a 5850. By about 50% or so, I believe (don't quote me on that, look at reviews).
More stream processors, at a higher clock, and a higher memory clock.
Personally I'd go for the 5850. It's plenty and will run pretty much anything currently out on max @ 1080p.
More stream processors, at a higher clock, and a higher memory clock.
Personally I'd go for the 5850. It's plenty and will run pretty much anything currently out on max @ 1080p.