Hi ! I'm not too sure if this problem should go under the Hardware forum, but since this happens only when I'm using KDE or Gnome, here goes.

So when I boot up Linux and receive system messages and enter my text login, everything's alright. But as soon as I startx, my monitor's image get shifted to the right.

I can manually shift it back left using my monitor's controls but it's not only annoying since I can't see scrollbars but I'll also need to shift it once again when I reboot my box. So I figure it could be a Gnome or KDE problem (X maybe?) but I'm not too sure. Or maybe a driver problem ? I just remembered I still haven't installed nVIDIA's driver.

Possibly useful info :
I'm using Slackware 9.1 with kernel 2.4.22. My graphics card is a GeForce 4 MX440 and my monitor is a Flatron F700B.

You can use xvidtune after you run startx to tweak your display so it is framed correctly. Once you get a result you like, note the settings (the show button will dump them out formatted for use in a modeline), then create a modeline for your X configuration file with your preferred settings.