Frame rate is one of the most important and one of the most misleading areas when evaluating a DVR. Please see the discussion in "fps" before taking as fact an advertised fps number. While many vendors provide accurate numbers, there are also many that do not.

30, 60, 120, 240 fps

Some DVRs specify fps in increments of 30,60,120, and 240. Typically, this refers to the number of frames per second the video capture board can capture. This is not necessarily the same as the number of fps recorded. Further, the fps number is meant to be divided by the number of channels available. For example, one might expect a 120 fps video capture board with 12 channel inputs to provide (120/12) or 10 fps on each channel. Often the actual recorded fps number captured (let alone recorded) is less than 10. The reason is that the capture board may spend some time switching between one channel and the next, resulting in a capture frame rate closer to 8 or 9.

Commonly Observed Frame Rates

< 1 fps  A frame rate of less than 1 fps is too slow for most real world applications. A second is a long time when viewing something in motion and it is likely that a system recording at less than one fps will not be adequate.

>= 1 < 4 fps  A frame rate of between 1 and 4 fps is extremely jerky, but if it may be a preferred frame rate when combined with a high resolution, such as 4CIF or D1. There is a balance between frame rate and resolution and often resolution is more important when reviewing an event.

>= 5 < 12 fps  A frame rate of between 5 and 12 fps is jerky, but adequate for many situations. A frame rate in this range combined with a high resolution, such as 4CIF or D1 is a very good system. Again, there is a balance between frame rate and resolution and often resolution is more important when reviewing an event.

>= 13 < 18 fps  A frame rate of between 13 and 18 fps will capture almost all the motion under most circumstances, but the motion will not be as smooth as television. It is certainly noticable.

>= 19 < 30 fps  A frame rate of between 19 and 30 fps is smooth by most people's standards. There are many DVRs claiming a full 30 fps that in reality are closer to 20 and most people either cannot tell or are generally satisfied that is television quality.

Automobiles and Traffic
A special note about automobiles and traffic. When viewing a moving vehicle, particularly when the camera is placed at 90 degrees to the motion, it is hard to capture without noticable jerkiness unless using a high frame rate (25 fps or better). The world is full of moving vehicles and everyone has become accustomed to their relative speed. The reality is that vehicles generally move very fast compared to almost everything else one regularly observes.
Test it
Testing frame rates is not difficult if there is an option to playback video one frame at a time. Simply record using the desired configuration for one minute and then count the frames for an entire minute. Be aware that if there is motion on all channels the system may not perform as well as if there is no motion on any of the channels.