Check this out --
Had similar issues with older monitors. If you have a TV to plug into give that a shot and see if it comes up. Not entirely sure what the fix is exactly, though -- using the method in that linked thread i was able to get the POST screen to change to a resolution the older monitor would show, but didn't change the resolution of the login screen to soemthing that i could see . However, if I hooked to another screen, booted, and then logged in, once I was in the desktop to set the resolution, i could unplug the newer screen and the older one would work (until reboot...). But hopefully that thread and the one that person links to helps.
