Solved: Ubuntu 10.04 (Lucid Lynx) boot up resolution
After installing the proprietary NVidia drivers for my graphic card, the boot screen resolution has changed to 640x480. It didn't look good at all the with the nice splash screens provided by plymouth. To resolve the problem I have added the following two lines to /etc/default/grub
GRUB_GFXMODE=1680x1050
GRUB_GFXPAYLOAD_LINUX=1680x1050
To activate the settings, use
sudo update-grub
Here the 1680x1050 is my screen resolution.
To select a nice splash screen, use
sudo update-alternatives --config default.plymouth
That's all