found the answer in: http://odin.prohosting.com/wedge01/gentoo-radeon-faq.html#4_refreshrate*_Question 4.5_*: /X is ignoring my modeline setting, or I can't get the refresh rate I want/
Try changing the "IgnoreEDID" setting in /etc/X11/XF86Config-4 to "on". Thanks.