Gegg:
First, is the monitor a VGA? If so, did you use a VGA to Mac adapter? A VGA monitor needs a Mac adapter. If you used two monitors to test the video signal, did you use a different monitor cables? Cables can go bad too.
If the onboard video is kaput, a PCI video card will work, as long as you use the proper driver.
As for the HD question, I'm not sure what your asking. If you mean, can you replace the hard drive, sure you can. If you only have one drive and the new drive doesn't have a system folder on it, then you'll need to install one from the System CDs. Be sure to backup anything you what to keep from the old drive. Or you can add the new drive as a second HD (making sure to set one as the master and the other as the slave).