Your encoder frame rate setting should be as low as possible to produce the desired result. Encoding all video at high frame rate is often just a waste of resources, and you can get higher quality video at lower frame rate in many cases.
This is because you have three variables:
Bit Rate -- how many 1's and 0's you will produce in one second
Resolution -- how many pixels there are in each frame
Frame Rate -- how many frames there are in one second
So if bit rate is fixed to, say, 500 Kbps, and resolution if fixed to, say, 640 x 480, then only the frame rate is left to adjust. A higher frame rate means the encoder must compress more data in one second than a lower frame rate, but because all values are fixed, then the encoder can only alter the quality of the image.
15 fps -- Talking Head Video, such as interviews, seminars, etc. especially when the camera is on a tripod
25 fps -- Good all around setting. Note that this is the standard for television in Europe (PAL)
29.97 or 30 fps -- Use for sports and other high motion video.
Note that because there is a direct relationship between frame rate, resolution, and bit rate, higher frame rates should be used for higher bit rates