Yes, you're right in thinking that it may be due to the graphics not supporting this particular resolution. From click here:
"1. Why my SiS Integrated Graphic can not support 1440x900 resolution?
If your SiS integrated graphic (SiS650 series, SiS661 series, SiS740, SiS741 series, and SiS760 series) can not support 19" WXGA for 1440x900 resolution, you may check some information below:
1. Update the latest UniVGA3 driver, you may download from Download Center.
2. Check your VBIOS version, go to "Control Panel"->"Display"->"Settings"->"Advanced"->"Administrator"->"Product information", and you can see the VBIOS information, please check if it is updated to 2.28.00 or later.
3. If your VBIOS version is not 2.28.00 or later, you have to update your BIOS, please contact your board maker or system provider to get BIOS update support."
If not already done, you can try their latest drivers from click here, choosing SiS UniVGA3 graphics driver. The latest is v3.9 dated 2009-06-26. G