I have used the X300 card, (a gift from my son - a gamer), happily for nearly three months without a problem in both XP and LINUX.
The other day in XP the screen went blank (black) and XP eventually rebooted.
After some experiments I discovered in LINUX that the card appears to be randomly switching H & V sync frequencies. In LINUX the desktop blacked out and the monitor came up with
 ... out of range
H:91.2KHz V:170.2Hz
so I switched to a console .. OK screen fine. Switched back to GUI and bingo! Screen back up again.
Had to do this several times then it settled down -- for five hours, yet.
Over the last couple of days the problem got apparently worse sometimes not even able to boot.
I upgraded the X300 drivers - no change.
I unloaded the drivers and am using XP's native drivers without any failures though screen refresh is really poor.
So. Am I looking at an X300 hardware failure? X300 firmware failure?
Motherboard failure?
Any clues, please?