There's always AMD. The open source AMD drivers are great for the 5000 and 6000 series, and support for the 7000 (and by extension 9000) series is getting better.
If you're interested in compute just pick up a 5850 or 5770 from a friend, they're cheap (if you can find them) and they run well with the open source drivers, and have support for at least 3 monitors. My 6850 has 2 DVIs, an HDMI and 2 miniDPs, I can drive 4 monitors at once with it.
I would counter that the AMD drivers are not great. I got a whitebox last year with a 5770, and the experience was terrible. The open source drivers couldn't do 3D acceleration properly, and the proprietary drivers refused to do dual-screen - the maximum available resolution across desktops was a square 1920x1920.
In my experience, nvidia has been providing a superior linux experience in the trenches for the past couple of years. The 'amd is good for linux' seems to be an emotion from yesteryear.
If you're interested in compute just pick up a 5850 or 5770 from a friend, they're cheap (if you can find them) and they run well with the open source drivers, and have support for at least 3 monitors. My 6850 has 2 DVIs, an HDMI and 2 miniDPs, I can drive 4 monitors at once with it.