I'd like to share this - just solved this problem a few days ago. Maybe it's helpful for someone else with similar hardware and X configuration.

When using X.org >=7.4 (xorg-server >=1.5) and the NVidia closed-source driver (177.xx) I needed to disable BackingStore to prevent X from crashing when KDE4 was starting up, or on many occasions like just moving the mouse to a screen edge when KDE4 managed to start up at all. Those crashes were accompanied by X error messages like the following:
[font=Courier]kwin: X Error (error: [DAMAGE+0], request: XDamageDestroy[DAMAGE+2], resource: 0x1401922)[/font]

You can disable BackingStore by adding the line
[font=Courier] Option "BackingStore" "false"[/font]
into the Device section of your xorg.conf.