Hard discs can have problems as the heads float on a small cushion of air and at higher altitudes this doesn't work. They are also very sensitive to bumps.
Solid state with memory cards is best.
You can get very acceptable results by using one of the still cameras that has a video recording mode.
Alternatively, look at
Helmet camera bullet cameras from dogcamsport for a range of products.The HQR2 is very good but around £600. They do also have cheaper units.
This video is taken with their gear
YouTube - Extra 330SC WAC 2009 training
YouTube - Silverstone National Aerobatics 2008 freestyle
Its difficult to judge the quality via YouTube, but we viewed the original on a 42 inch flat screen and it was very good.
ZA