Have you tried other monitors?
Does it boot all the way even with a blank screen? You can try turning on file sharing when you get a working screen then wait the normal boot time to access your G5 over a network with another computer.
It could just be a failing video card. Do you have the hardware diagnostics CD? When you do get video on boot you can try running tests with that.
Beyond that the only other way I can see testing the video card is getting a cheap replacement off ebay and trying that out, get a PCI one for testing it should be cheaper than any AGP for G5's. It would certainly be cheaper than a technician.
