I have tried running redhat-config-xfree86, and also tried to edit the /etc/X11/XF86Config file. I do see higher modes in XF86 Config file, but the system defaults to 640x480 configuration, no matter what change I make in either case.
Looking at the XFree86.0.log, it appears to me that the system sees only 832 KB memory in the display adapter, although it has 16 MB memory. Hence, it always assigns the lowest resolution, as it thinks there is not enough memory for running higher modes.
Can someone suggest me what I should do next to fix the problem.