I had a problem like that with my graphics card. When I moved my computer to the other side of the house my computer would turn on and after it got past the xp loading screen the monitor would turn off like it was in sleep mode. I thought my video card was broken. So I took my video card out and moved it down a slot...since it's pci and not agp. Then I started in safe mode, reinstalled the drivers and restarted and it worked. If you look on the back of your monitor it should say how many hertz it can run at. Most monitors can run at 60.
Your best bet would to change your resolution to the lowest it can go. Since you are running with your old video card you can change it down to like 800x600. Then put your new video card in, start in safe mode, install the drivers and restart. That should work.