you have to use a the sis miniport agp driver... 1.14 is the latest.
When in doubt RTFM!!!!
btw the sis 645dx and 655 chipsets are great performers unless you happen to be unlucky enough to have the asus board you do. Do a search on any tech based bord and you will find many a hundred unhappy users.
Your best bet for help is asusboards.com(not affiliated with asus) message board. lots of knowledgeable people who will be more than happy yo help you out.