I used to run an SiS 6326 AGP card in a box with Redhat 7.3, so you should be able to get it to work. I don't really recall what I had to do to get it running, as it was my first Linux fileserver, dns etc. and that was a while ago. I did have X running on it though, so I thought I would let you know that it can work.
Have you checked out Thomas Winischofer's site? http://www.winischhofer.net/linuxsisvga.shtml