When constructing a modeline for use with the Sis driver you'll need to consider several points:
SiS recommended video modes for 6326:
4, 8, 15, 16, 24 bpp at 85Hz Non-interlaced
4, 8, 15, 16, 24 bpp at 85Hz Non-interlaced
4, 8, 15, 16, 24 bpp at 85Hz Non-interlaced
4, 8, 15, 16, 24 bpp at 75Hz Non-interlaced
4, 8 bpp at 65Hz Non-interlaced