Well, I've been having problems with radeon. My card (Firepro v4900) is overheating when a dual screen setup is in use. The reason seems to be pretty well outlined here: https://wiki.gentoo.org/wiki/Radeon#Power_consumption_when_using_multi-head_.2F_multi-monitor
And my old friend, dpm, is to blame... Luckily, I got another v4900 from a friend and plugged it in.
Oddly, X seemed to recognize it, but GNOME would not see the display.
Well, some xrandr magic was to be conjured:
xrandr --setprovideroutputsource 1 0
This did it. It is telling card1 to use card0 as it's source. After that command, GNOME can see it and overheating goes away.