Whenever you launch the launcher you should have some graphical settings and you should see a row that says either disabled,2x,4x,6x,8x, or 16x. Do not exceed 4x on that option or it will crash. Hope this helps!
That depends on the graphics card. I've tested at 16x and it just runs slow.
"Disabled" is the best option for compatibility, though.
Oh must just be me

For some reason, that dialog window seems to give users options that they can't possibly use... I'm not sure there is a good way to sort through all of the unavailable options but someday I will try.
Is it just amd devices having this problem? Because I find it quite hard to believe that my 6950 can't handle more than 4x

Not sure. I'd have to see the error.
Uncaught exception from thread:Thread[LWJGL Renderer Thread,5,main]
org.lwjgl.LWJGLException: Failed to find ARB pixel format 1 0
at org.lwjgl.opengl.WindowsPeerInfo.nChoosePixelFormat(Native Method)
at org.lwjgl.opengl.WindowsPeerInfo.choosePixelFormat(WindowsPeerInfo.java:52)
at org.lwjgl.opengl.WindowsDisplay.createWindow(WindowsDisplay.java:210)
at org.lwjgl.opengl.Display.createWindow(Display.java:306)
at org.lwjgl.opengl.Display.create(Display.java:873)
at org.lwjgl.opengl.Display.create(Display.java:782)
at com.jme3.system.lwjgl.LwjglDisplay.createContext(LwjglDisplay.java:137)
at com.jme3.system.lwjgl.LwjglAbstractDisplay.initInThread(LwjglAbstractDisplay.java:113)
at com.jme3.system.lwjgl.LwjglAbstractDisplay.run(LwjglAbstractDisplay.java:205)
at java.lang.Thread.run(Unknown Source)
Application was not initialized.