Interesting. Looks like there is a bug in the sis driver in Ubuntu Intrepid 64 bit.
This HAS to be the reason you are unable to get this working, not that you are doing anything wrong, just Intrepid has a broken sis driver.
Since I reported this bug I had to change to Xubuntu 8.10 32-bit, where a sis display driver is available and working quite solidly, will try to get a little time post it on my site,
Looks like you are at the mercy of the Ubuntu bugfix team, and apparently they didn't feel the sis driver is all that important.
Makes me appreciate Debians dedication to bug fixing before releases.