I have Asus P4S533-MX and done everything you told to get wide screen support.
The problem is I get only 4:3 resolutions with SiS driver (UniVGA3 3.73) installed.
But when I uninstall SiS driver (UniVGA3 3.73) i can get widecreen (1440x900)
While flashing I got only the option "Update BIOS without Boot Block and ESCD" instead of "Update BIOS including Boot Block and ESCD"
Is that anything related to the problem?
One more thing I am using LG LCD TV as monitor (22LG30R)
Please refer the following screen shots, awaiting for ur reply