Re: Old member...New build
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.
Desktop i5-4440 - ASRock Z77 Extreme4 - EVGA GTX970 - 4x4GB 1600MHz - Samsung 850 EVO 250GB - Be Silent Base 800 - 3TB+1TB+640GB
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Uni MSci Ethical Hacking, Coventry University
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LinkedIn Kiera Mitchell