Just a quickie to let any other poor victim with a S3 ProSavage DDR card on their pc/laptop know this.
If you've got problems and when you test your config it gives a garbled screen then pick the 86C764 (Trio64) on install at 16bit (65,000 colour) 1024x768 and bingo it works
All the other settings I tried ( a whole day trying in fact ) and this was the one that worked.
Once working you can follow the instructions for upgrading on the S3 page at Xfree86.