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Interesting article I stumbled across.
-By Theo ValichA source close to AMD told us that there will be inter-generation compatibility between the Radeon 3800 and 4850: Since they're both DirectX 10.1 parts, there is no reason why those two should not work together. ( ) We will probably not qualify this interoperability, but who stops you from trying?"
It appears that ATI did not integrate a driver lock or something else that would stop you from combining a 3870 card with a 4850 or with a 4870 model. Just keep in mind that these two cards need to include the same amount of memory. If you mix and match a 3850 with 256 MB of memory with a 4850 512 MB, the 4850 card will default to 256 MB. If you wonder whether you will be able to combine a 3870 X2 card with a 4850 board, our sources indicated that this scenario should work as well.
edit #2: looking in the comments on that article, someone linked an article at fudzilla saying that this is not true...so...draw your own conclusions.