Hi
Take this with a pinch of salt.
I expected Slackware 9.1 (XFree) to auto config my card. It didn't.
Then installed Slackware 10 (with X.org). It did.
In my xorg.conf (your xf86config file, you should have something like device "sis".
Also see:
http://www.winischhofer.at/linuxsisvga.shtml.
He is the maintainer/contributor of SiS driver for x.org.
it will give you detailed config for your SiS card.
bye