Using different drivers for the current video adapter may enable it to display at 640 x 480.
If that does not work, then installing a video card that offers 640 x 480 will solve the problem.
Chack if your current video adapter supports dual monitors.
If not, then you need to install a video card that does.
But running one monitor at 640 x 480 and the other at a higher resolution(or both at 640 x 480) is not the way to go.
Suggest that you replace the Panaview 540 with a newer monitor that can display 800 x 600 or higher. 17" crt units are inexpensive, and 15' crt's are even less.