Xwindows problems

[Andy]

hello, I was wondering if u could help me with Xwindows.  My system is running redhat linux 5.1 (the free version) and is a pentium 90, 24mb ram and a 1mb ATI mach64-ct graphics card.  The problem is this:   I start Xwin through linux using the Startx command, and xwin starts up, but the I cant see anything on my screen because the screen res is far to high for my monitor.  My monitor is a 15" 640x480 compaq monitor.  I setup Xwin through the xconfigurator program that run after linux was installed, i choose a generic monitor and all of the colors (8 bit, 16 bit and 24 bit) to run as 640x480.   if u can help me get xwin to run at 640x480 res i would be grateful, thanks, andy