This solved the problem...Found it on a post on this website!! Thanks a bunch guys!
I have solved it, almost miraculously. After a google search for a driver update for the S3 ProSavage KM133 video card, I stumbled upon a site that explained why this problem occurs (at least with this video card)...
It happens to be that when using this card as an integrated chipset into VIA motherboards, and using shared DDR memory for the card, the BIOS of the mobo messes up some key configuration elements, overriding the XF86 configuration file.... Don't ask me for specifics, all I know is it works now, after adding the simple line
Option "UseBios" "no"
to the "Device" section corresponding to the video card. (In the XF86Config)
So that's it... hope this could be of any help.