I see no one has jumped in, so while not really a Mandy expert,nor do i use a LCD monitor i can only offer some general suggestions..
First of all it seems a lot of people have some problems setting up a LCD monitor..One of the problems seems to stem from the fact that
the 1280x1024 aspect ratio is 5/4 rather than the 1024x768 which is a 4/3 aspect ratio(old crt monitor standard)..That said I think the most important reason that you cannot get the 1280 resolution is because you somehow you do not have the correct scan frequencies entered in your xorg.config file..
My advice would be to check your book monitor specs and enter the correct scan frequencies if they are wrong..Do not go over
the monitor frequency in either the Horizontal ot Vertical frequency as this may damage the monitor..
I am not sure how you do that in Mandriva but I know in suse they can use sax2..Maybe someone with more hands on Mandriva experience can jump in and help..
So here it is:
1 Enter the correct scan freq. for your monitor in the xorg config file..
2 after you get a stable screen at your desired resolution try to change the aspect ratio and screen size so as to not get an overscan or underscan resulting in black areas or extending beyond the screen..
3 it would also pay to install the correct driver for your video card/chip..I am not sure whether to do this first of later..Maybe someone else can offer a better suggestion...Feel free to jump in..
Thats about all I can think of now and it will require you to tinker a bit..So....