First off... from a commandline sys... install package hwinfo
sudo apt-get install hwinfo
Post all that info. Also post the make and model of your old Monitor.
sudo hwinfo --framebuffer
sudo hwinfo --monitor
lspci -vnn | grep -i VGA
Here's what goes on from all that- Your old monitor, that you say only supports 640x480 us probably too old to contain internal EDID data... So before you create an xorg.conf file for your system, you need to know what modes the monitor supports and the modes your video card supports at a certain scan rates... To create modelines to place in the xorg.conf file to recreate that missing data. Dependng on the video card, that configuration file can also point to the pertinent video driver.
If you really want to do that yourself, go to the 2nd post in my Graphics Resolution sticky (link in my sig line) and look for the modeline tutorial links... Or I can just create it all for you from the info you post.