The original handheld line was officially the
iPAQ H3600 series; individual models differed in size of the (
flash) ROM and RAM as well as he display (monochrome vs. 4096 colors). Newer models have a higher color resolution, built-in
SD card slot and
Bluetooth support.
The iPAQ is famous for being the first PDA capable of running Linux, and it does so quite well; there are several distributions to choose from.
Unfortunately, although the Linux capability was planned for and is strategically supported by the maker, installing Linux in the device's ROM is a rather scary process that can potentially render it unusable. Furthermore, it requires a serial synch cable that doesn't even come with the device.
I'll probably stay with WindowsCE, cuz I got a Japanese model which has ultra-cool kanji handwriting recognition built in.