The H3630 and H3650 are the origional iPAQ models. The h3630 was the version sold in retail stores, while the h3650 was to be sold through online stores. Because of the scarcity of iPAQs the distinction broke down, and both models were sold online and offline. There is no difference whatsoever between these two models. Retail price $499
The h3635 is the most reacent version of the iPAQ. It is identical to the h3630/50 models. Packaged with it is an additional compact flash expansion sleave and several pieces of software. Retail price $599
the h3150 is a low end model with 16mb of ram and a black and white screen. retail price $350
the h3670 is a soon to be released 64mb model. it will oterwise be identical to the h3630/50/35 models. retail price ~$700 (not yet final)
the h3635 models will be newer production iPAQs. they will most likely carry a serial number beginning with 4g12 or higher and have significantly less problems with dust and other defects.
I love the iPAQ. it is by far the best, most versitile handheld device in existance. I have owned or used a Jornada 548, a cassio E-125, an iPAQ h3630, and i am currently using an iPAQ h3635.
this is the best forum on the internet for iPAQ related topics.