I have a Matrox G200 Millennium 8MB AGP and a Sony 17" 200ES monitor. During probing/testing of 1280x1024 mode (any colours), X makes the monitor looses syncs, and hangs any video output (also in text mode), so it's necessary to do a "blind" reboot. Same thing happens setting-up manually with xf86config ...
Do lower resolutions work?
Probably - should also be fixed by new DDC-probing.