I believe that you can choose your own Refresh Rates(within manufacturer's limits though) using a proggie called gtf
I shall explain it here..
Say for example, you want it to be 640*480 @ 75Hz.

Install gtf (dunno how its done on RH

)

You can use the following command : gtf <vertical size> <horizontal size> <refresh rate>

It will give an output as 2 lines...
Code:
# 640x480 @ 75.00 Hz (GTF) hsync: 37.65 kHz; pclk: 30.72 MHz
Modeline "640x480_75.00" 30.72 640 664 728 816 480 481 484 502 -HSync +Vsync

Paste these 2 lines in your xorg.conf(dont forget to backup) in the monitor section
Code:
Section "Monitor"
Identifier "Generic Monitor"
HorizSync 31-101
VertRefresh 60-160
EndSection
will now become
Code:
Section "Monitor"
Identifier "Generic Monitor"
HorizSync 31-101
VertRefresh 60-160
# 640x480 @ 75.00 Hz (GTF) hsync: 37.65 kHz; pclk: 30.72 MHz
Modeline "640x480_75.00" 30.72 640 664 728 816 480 481 484 502 -HSync +Vsync
EndSection
Hope it helps...
BTW Source from an Ubuntu support forum...
Cheers!!!
] K8)8)L [