helmetcameracentral.com/ and
www.helmetcamreview.com/CamTests.htm have a heap of reviews on most helmet cams. This can help narrow down your search. You can generally download full quality sample videos from most of the manufacturers website, or search on vimeo.com or something.
Another thing that may be a better option is something like the Panasonic Lumix DMC-FT1. It shoots video in 1280x720 at 30fps. You can also shoot at lower levels and store more footage, and it will take decent photos, something that a normal headcam won't do. Although you'll have to figure out how to mount it and also stop/start recording easily.
I'm not sure about the other waterproof digi-cams. I know the older Olympus cam's only done 15fps at their highest resolution. Not sure about the new ones.