How to center a 1024x768 framebuffer on a wide screen ?

Hello,

I have a laptop with a 1280x800 native resolution (Dell D430 - intel chipset)
With the Knoppix Live CD, I get a 1024x768 framebuffer resolution which is not stretched, the screen is centered.
How do I get this using another distribution ?
The screen is stretched (in horizontal due to the wide screen) everytime when I use vga=791 in the grub.conf file.
I checked all parameters of intelfb and vesafb framebuffers but nothing about centering screen.

Thank you for your help. Cheers !