I have managed to get the SIS671 driver working by making some changes to the source code & re-compiling it! I am now running at 1360 x 768. Not quite the maximum resolution (1366x768) but hardly detectable...
I have created a new Special Timing option for the ASUSX5DC. I then edited, xorg.conf and under device section I placed the following parameter:
Option "SpecialTiming" "ASUSX5DC"
I need to do a bit more work to the source before it is ready for release(I will be asking for the SIS671 Driver to be patched); in the meantime, if you have a ASUS X5DC and you would like a copy of the driver for test purposes, I am happy to provide a copy! Just leave a comment against this blog or email me, and I will send you a copy.
I have also extracted the EDID information from Windows 7 Registry and beginning to decode it. I hope to modify the SiS driver to read the EDID information directly from a file, enabling anyone to port EDID Information from Windows Registry... Useful if a manufacturer used the Microsoft's Windows - Custom EDID option, when configuring the screen driver.