I have a fresh install of Kubuntu 15.04. Before installing 15.04 I used Ubuntu 14.10 with the latest NVIDIA drivers. With that version I had no problems with the graphic card (screen resolution). Btw. my PC contains a GeForce (GF104) GTX 460.
As far as I know the default installation of Kubuntu uses the NOUVEAU driver. I selected the driver 346.59 provided in System Settings -> Driver Manager to switch to NVIDIA. (I even tried the older versions)
After a reboot I had a very bad screen resolution. I checked the settings inside "NVIDIA X Server Settings". There I have seen that the current resolution is: 1920 x 1080, which was not correct. The resolution looked like 640x480 or sth. like that.
Even the "Display Configuration" of Kubuntu showed me that value. I tried to blacklist NOUVEAU but it didn't change anything. Even the "boot screen" of Kubuntu disappeared after switching to NVIDIA.
So what am I doing wrong?
Update: This is what I get after booting with NVIDIA driver.
dmesg | grep nvidia [ 6.700837] nvidia: module license 'NVIDIA' taints kernel. [ 6.714403] nvidia: module verification failed: signature and/or required key missing - tainting kernel [ 6.719445] [drm] Initialized nvidia-drm 0.0.0 20140818 for 0000:03:00.0 on minor 0