What is the Top Speed of X bike.
When you are talking about offroad motorcycles, the "top" speed may not really be important unless you are going in the Baja or going out to the salt flats for a land speed record!
Most modern motocross bikes will have a lot more speed available than the average rider can fully utilise.
If you are Ricky Carmichael, Bubba Stewart or Chad Reed, then you might need to wory about whether the bike can get you around the track fast enough, but that has little to do with the "top" speed.
What it does have to do with is their ability, the motor, gearing and suspension of the bike. Overall, the "handling" of the machine.
Top speed is really only relevant if you will be riding on the highway at what is probably illegal speeds, and can easily be varied by simply changing sprockets.
See http://www.dirtbike.ws/node/4679 for more information on changing the rear sprocket.
See http://www.dirtbike.ws/node/4681 for more information on gearing ratios