help for newb with supposed graphics card problem

Please help

My friend recently experienced problems with his computer. He returned from classes one day and his screen had an odd message on it (something to the extent of "monitor power on- check signal from pc" ). He checked his monitor connection and it was fine. He then proceeded to inspect his graphics card thinking it was malfunctioning. He took out his old GeForce 2 MX and then bought a new Ati 9800 pro thinking it would solve the problem. I helped him install the card and when we tried to boot up the pc, the same message was displayed on his screen. We heard that PCI cards might interfere with the installation so we removed the NIC card and the modem and tried to boot it to no avail. I don't even know where to begin to fix this problem with his machine. Any suggestions would be greatly appreciated.

Thank you in advance,
Chris

Thank you for your reply. I will try to provide some more details:

The power supply is good as all fans (cpu,video card) were spinning and a green led glowed on the mother board when the computer was started. It might be his RAM (he has one stick of 256 pc2100)

I don't think it is a monitor problem. We tried 2 different monitors (we know the second one is a good unit) and the same message was displayed on both. I hope his mobo isn't kaputt. That would just suck.