I found the same situation when I brought the new LCD 20" and My Mainboard was very old and cannot change resolution to widescreen. Please pacth the new SIS VGA Bios for my old mainboard's bios.
I cannot post url link for download bios file so I will give you for key word in google instead.
"BIOS for L4S5MG/GX+(PCB:3.02)"
**lastest bios version is 1.0e**
Thank you in advance for your kind.