Monitor Drivers

I have an AMD 64 bit computer with a Saphire radeon x550 display card with which I run the supplied ATI software and I have also tried other variations of ATI software downloaded from the internet. I have an old 19" supermatch monitor that works best when driven at 1024/768 70Hz When I use XP64 beta or WIN2000 the ATI setup tells me it is running the monitor at 70Hz but actualy puts out 60Hz which results in bent verticals. I have downloaded beta versions of Solaris,Vista and Linux and they all drive the display at the correct 70 Hz but I wish to use XP64 has anyone else had this experience or knows where I can get an ATI version that generates 70 Hz