[all variants] Cannot change resolution on laptop

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http://ubuntuforums.org – Hello forum, I have Kubuntu 9.10 running on a Dell Vostro 1720 with 17" display with a resolution of 1920x1200. The graphics card is a GeForce 9600M GS and I am using the nvidia driver for it. At 1920x1200 everything works really well, my problem is that I can't change to a lower resolution. I've tried adding new resolutions in xorg.conf, but that just resulted in warnings like (WW) NVIDIA(0): No valid modes for "DFP:640x480+0+0"; removing. in the Xorg.0.log. I've also played around with xrandr, but there I could not add new modes, because I fail (Hardware)