I have a Sapphire 3870 512MB and a Sapphire 7870 2GB. Can I Crossfire them?
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I don't think you can. I'm not sure about older cards, but for AMD's new cards, you can only crossfire a 78xx card with another 78xx. So a 7850 and a 7870 would be fine, but a 7850 with a 6850 or a 7950 would not work.
You could only crossfire a 3870 with another 3870 or a 3850.
A 7870 can only be crossfired with another 7870 or a 7850.