..And what does 60Hz & 100Hz actually mean regarding the monitor?
The Hz frequency of a monitor is autonomous from the frame rate. It is an internal refresh rate of the picture on the monitor. Usually refresh rates for CRTs were sat to 60Hz. But when you use CRTs from 17'' onwards, you might notice the monitor picture itself is flickering a bit like on an old CRT-TV. You can note this if you stare at a point next to a screen while concentrating on the picture/screen. Low refresh rates might cause headaches then. If you raise the refresh rate from 60Hz to 75Hz now, the flickering will vanish, thus being more compfortable for your eyes, especially when sitting longer times in front of the screen.
The ability to have a high Hz refresh rate depends on the quality of the monitor. Quality CRTs can run resolutions of 1920x1440 at 100Hz, while others can only run such resolutions at 76Hz or even 60Hz. In general the Hz refresh rate of a CRT drops when raising the resolution, thus all CRTs can run 1024x768 at high refresh rates, while only to good ones are able to display high resolutions with high refresh rates.
TFT monitors generally run at 60Hz. Raising the refresh rate will give you a black screen /out-of-range error. You should keep running TFTs at 60 Hz unless stated otherwise. The system used here is different, unable to cause flickering.
In my case, running a high resolution with very low Hz refresh rates on my 17'' CRT, causes extrem eye cancer. *painfully rubs his eyes*