The older Intel chipsets don't have support for widescreen resolutions. Had to repeatedly bang my head before realising that the resolutions are hardcoded in VBIOS, they don't read the Monitor EDID directly. They updated the 965 BIOS to include 1440x900, but not 1680x1050.
Stupid idiots at Intel. Not that I use the onboard graphics myself.
To OP: Try a BIOS update, you may get lucky but IDK if they support the boards anymore. There are tricks to get it but you have to go registry hacking and manually editing the EDID information in the driver system files. Not a pleasant thing to do.