I did a fresh install of Ubuntu 11.04 on a HP 550 Notebook. But grub2 isn't really fit in the best screen resolution that this Notebook can support (1280x800).
How can I do this without using a third party application ?
To do this safely requires two steps.
Step 1: find the preferred mode
Reboot and press and hold
this will display various stuff how grub recognises your display. At the bottom is "preferred mode" - in your case it should say 1200x800. Note down the value.
Note: sometimes, some buggy video cards incorrectly give Grub the wrong preferred resolution - if the preferred mode is much higher than you were expecting, then select the nearest mode in the list displayed that you were expecting.
Step 2: Setting the resolution in grub
Reach for your terminal and type
find the line
remove the # and change 640x480 for the preferred mode you wrote down.
save, then type
Easy to use Grub2 settings customizer. Go to "Preferences" -> "Appearance"