How can I set 1680 x 1050 on GMA 900?

Specifically that option does NOT usually come up. But 2048x1536 at 85 Hz maximum resolution is the max available so your size should be no problem. You may need the proper driver. It should have come with the monitor. If not you can try and Uninstall/Remove your video driver. Then just reboot. Many times the system will now search for the best driver. Then tell it to search your install disk. Many times the disk gets installed first and THEN you remove/reinstall.