Here's my two cents: I've used linux since 2007. I used to be a die-hard Radeon fan (and for Windows-only users, I might still lean that way). But for linux, I cannot recommend _ANY_ Radeon card. AMD, for whatever reason, does not support their even slightly older cards with the proprietary drivers. Whereas Nvidia does. AMD actually goes out of their way to turn off driver support for older cards. I first experienced this with a Radeon 4850 that worked beautifully, but AMD decided to not include the 48xx series in its drivers anymore. There are tweaks and ways around this, and I put up with it for a while. But decided no more; for linux I only recommend nvidia.
I just sold a 6870 on eBay -- no kidding -- because I was sick of dealing with AMD driver stupidity. My replacement (soon - holding out for a while for possible holiday deals) will be a Zotac GeForce 750 Zone -- great card, and passive cooling. Nvidia's new cards focus on power efficiency and low temps, which I totally agree with.