I've heard people prounounce this about five different ways (i.e. how-pawje, hop-idge, hap-oh-gie...). Anyway, from the horse's mouth:
How do you pronounce Hauppauge?
Hauppauge is a town in NY on Long Island. Hauppauge is pronounced HOP-HOG. The area around the headwaters of the Nissequogue (NISS-I-QUOG) River was dubbed Hauppauge by Native Americans; it means "overflowed land'' in the Algonquian language. Hauppauge boasts a large industrial park which is home to many well known companies.
- from http://www.hauppauge.com/html/faqusb.htm#HOPHOG