Re: Restarted my machine, now suddenly NOTHING works with my GTX670?
Well at the bottom of your xorg.conf you have an option to disable the composite extension. I think this happens when Xinerama is enabled. I don't think Xinerama works with compositing.
Also, what driver version are you using?
I'm not sure if this is what you want to do or not, but you should be able to use TwinView with all of your monitors... but you might need a 30x-series driver.
Separate X screens are probably workable (though I have no experience with them), but I think Xinerama is universally scorned upon these days, if not in practice deprecated.