Comment 4 for bug 584339

http://en-gentoo-wiki.com/wiki/Framebuffer helps sort out
what to expect with SiS, and how to set things right.

With Lucid I get my best performance by adding the following:

'video=sisfb:mode:1024x768x32,rate:70,mem:4096'

between 'ro' and 'quiet splash' in the grub menu entry.
Leave off the quote marks, be precise with punctuation.

Hope that works for others.