[all variants] Crash, possibly GPU; which log files?

I get some crashes but I have no idea which log file to have a look at, since there are so many, both in /tmp and in /var.
I suspect the cause is the GPU (but I'm not sure), so I should probably focus on those logs, wherever they are.

System
I use Xubuntu with Xfce (as it is by default).
Radeon HD 5770.

Crash
Sometimes, usually watching YouTube, the screen gets kind of dark green with probably some vertical lines.
The last second of audio gets played in loop for a while, and then it stops and I have to hard-reboot the PC.