There are a number of different "helmet cam" setups that you can buy. They typically have a small camera head that connects to the controller in a backpack. They are not cheap.....
I have taken some decent video using a cheap camcorder that I simply duct tape to my helmet. You can purchase some fairly decent units for under a $100.
What you want:
30 FPS (frames per second). Anything less is just not watchable when you are moving fast. Cheap units may only provide 15 fps
640x480: Cheaper units will usually drop to 320x240 pixels when they go to 30 fps.
Store to SD memory: A 2 Gb card will provide an hours worth of video.
The unit I have doesn't have any zoom or focus, which is OK since I can't operate the zoom / focus when I am riding anyway.
Rod