Do you still have that Dell diagnostic? Just a reminder that your previous errors were graphics/drivers related.
Just to clarify: when do you get the "Cannot Display in this Video Mode" error? After the BIOS? Before XP loads?
Is the only way into XP thru Safe Mode? Have you checked the card in the Device Manager? Looked for errors there & or update drivers from there?
Perhaps something here will help:
http://www.google.com/search?q="Cann...0Video%20Mode"