I've always had problems with the i810 on-board graphics and Linux. Last I tried (with Mandrake 8.1 and RedHat 7.2) neither worked with anything other than 16-colour VGA. Not Nice. Fortunately these were test PC's at work and I got round it by simply not using those ones for my GUI linux projects!
Have you tried Mandrake 8.2?
In theory if it's let you install KDE it would have installed all the necessary dependencies including X, xdm, the font server etc etc. It should also boot to the GUI with xdm by default, so if it's going straight to the command line (init 3) then something's really wrong. Try logging in as root and typing "init 5" which should take you to the graphical login.
If it doesn't even go to a VGA screen, try logging in as root and running "Xconfigurator" to reset your settings. Go for normal VGA at 640x480 to start with, just to make sure it works.
If that works, re-run Xconfigurator and go up one level 800x600 and try again. You might be able to get it up to your desired resolution, but if you go up in stages you'll know the highest one that works.
Good luck!