Usually objects falling 'from infinity' have speeds near the local escape velocity. For the inner solar system, this is about 30 km/sec very approximately. This means that Hale-Bopp is probably moving at near this speed within a factor of 2 or so. Sorry for the ball park estimate, but none of the HB sites I quickly visited had this information. By the way, 30 km/sec is about 70,000 miles/hour.