Is the monitor cable snug with the graphics port and the monitor?
Is there another graphics port on the motherboard you can try? (probably not but I don't have much experience with on board video)
Have you tried the monitor on another computer to see if the monitor is ok?
Maybe try taking a video card out of another computer and try it in yours?
If you determine its the onboard video that is the problem then you can buy a cheap PCI-E/PCI/AGP card for like $20 if you dont need a high end one.