From what I have read with Crossfire both cards have to have the same chip. That is why you can Crossfire a 7950 with a 7970 they are both the same chip. I can't say about the 270x and the 7850 as I have not read up on the new AMD cards very much. But from what I could google yes you can but the 270x would just run at the speed of the lower card,the 7850. So if you could find another 7850 that might be a better idea for a Crossfire depending on the price of course.