Well, as a rule of thumb, a refresh rate of 85Hz is best for most people as it removes any noticeable flickering in most circumstances.
Setting too high a refresh rate then your monitor can handle can damage your monitor, so it is always useful to check your monitor's manual beforehand.
Personally, I prefer 100Hz as my refresh rate at 1024x768 and my monitor can handle it. However if I bump up by resolution to 1280x1024, my monitor can "only" handle 85Hz. Perhaps it is because of the lighting in my room or my sensitivity to flickering, but I find anything below a 100Hz refresh rate to be a strain on my eyes. However, this on the desktop, in games, flickering is much less noticeable. But still, I would recommend nothing less then 75Hz if you don't want eyestrain.
Also, if you no longer have your monitor's manual, you can always check the display properties from the control panel and if you have the correct monitor drivers loaded, it will tell you your monitor's limits.