Which ATI card do you have?
Which Distro are you running? (Fedora, Ubuntu, Arch?)
http://support.amd.com/en-us/download/linux
SHORT VERSION:
Older cards are usually supported "out of the box" on linux.
But high end options like SLI and Crossfire aren't.
You would need the ATI/AMD Catalyst/Crimson driver. Called FGLRX in linux. "Fire GL & Radeon for Linux".
The newest is Crimson 15.12, AMD is working on a new driver setup.
(But they aren't updating Crimson anymore, see next sentence.)
AMDGPU which is the new open-source kernel side driver, which lets the proprietary AMDGPU-PRO run 'on-top' of and 'plug-in' to the kernel side.
(So kernel updates won't "break" the driver.)
amdgpu handles all the base stuff, would only need PRO (still beta) if running Crossfire, etc.
It will eventually (the're still coding this) work for, R7 300 series, and R9 200, and RX 400 series and newer. No pre-GCN cards on this one. (but hey, day 1 driver support for linux.)
I'm running 2 PCs with Fedora 23... well, The AMD 5600x2 has an 40GB IDE with Fedora 20, and Catalyst 15.4 (I think), and a SATA with Fedora 23 with Crimson 15.12 (catalyst rebadge in linux).
This machine runs dual 6750s in crossfire... but the driver works with older cards too.
Check the readme with the drivers.
My FX8350 is running dual R7 370s in crossfire on Fedora 23 with Crimson 15.12 also.
I Know that these are much newer, but newer hardware is usually what doesn't have driver support. Older hardware is more often supported.
Let me see if I can find the page that has what all the open source Radeon driver covers.
Barracuda
Almost forgot, There is a setting in the Catalyst/Crimson driver, where you can set to "maintain aspect ratio" or "scale image to full screen".
if you can tell me which Distribution your are running we can nail down where it is in the settings, or how to get FGLRX installed on it and get you back in the game!