Are you trying to get an xorg.conf that will work with whichever of the two monitors you choose at boot time? Or are you trying to just make it work at boot time with the new monitor?
What resolution do you want to use with the new monitor?
Your xorg.conf is strange because:
50 is a low maximum for HorizSync. That will generally limit you to 768 scan lines, yet you have a lot of modelines in your monitor section many of which have more than 768 scan lines.
Your Screen section limits you to mode 1024x768, but in the big list in your monitor section there is no modeline for 1024x768, so when X starts it needs to find a 1024x768 modeline somewhere that is compatible with that low HorizSync value. I think that is the key to your reported symptom. One place X can get a modeline is from the firmware of the monitor itself. Your old monitor probably provides a 1024x768 modeline that is compatible with your HorizSync value.
Your new monitor works with that modeline (I assume not as well as it would work with a better modeline) but your new monitor may not offer a 1024x768 mode at all and if it does it wouldn't offer one compatible with your low HorizSync value.
If you just want the new monitor to work, change the HorizSync and VertRefresh ranges to be correct for the new monitor, and probably add a modeline for the mode you want to use for the new monitor and if that isn't 1024x768 edit the screen section to reference that mode.
If you want either monitor to work from boot at the slow 1024x768 it is probably getting from the old monitor's firmware, use videogen or similar tool to construct that modeline and put it in your monitor section.
If you want either monitor to work from boot with a mode appropriate to that monitor, sorry I don't know how. I'm sure it can be done, but I'd need to try several different things to see what does what to find out how.