You don't need 100Hz for gaming as 85 or 75 will be flicker-free. Have you had this monitor run at 100Hz before? I wouldn't be surprised if a refresh rate that high has been dropped in Windows 7 given that CRTs are old hat and TFTs don't go that high.
anything over 75 is more than you can see. i do recommend setting to the highest you have available as it helps reduce eyestrain, but that is a long term effect not one that you can perceive at any point
WIn 7 supports rates over 100hz. I have a 120hz Samsung 2233 lcd panel and 3D glasses......
Your ati drivers might not support higher refresh rates since they don't have 3D support.
Only nvidia has that right now with a driver addin.