minimum cap on ur main should be 25 fps. Ur system should handle it perfectly fine, but capping it at 20 is going to cause you some problems. whilst display frequency is the important one to avoid flickering (potentially nauseating and in extreme cases can cause epileptic fits in those prone to it), having a slower frame rate then 24 over a long period of time can cause nausea amongst some people. Having the Brain readjust to spacial positions (even if they are fake - ie flat pc screen) for long periods of time is somewhat unhealthy :S

I would suggest 30fps personally. If you plan on recording any of your in game footage you should stick to 30 (29.9) fps, as lower then that, depending on the video capture or editing software can cause the end fps to drop below 24 fps (but that usually isn't a problem when making .avi vids, as it applies mostly to NTSC