At the start I wanted one just to run a specific piece of work related database software, and looked at the most basic ones. After all I am a smart person, right? I don't need one with flashing lights, GPS recievers, a built in cell phone, wifi blue tooth.
In the end my company presented me with an iPaq Rx3715. Its a seriously handy piece of kit. The Wifi is mega handy when on the road. Instead of hauling my laptop, it gets left at home as the iPaq is more then adequate for brief internet surfing and e-mail checking, even ppruning!
The GPS is actually something I have realised that I wished I had now too. On a recnet trip to frankfurt a colleague set his up in our hire car and it took the navigation burden off and was super usefull over the few days in an unfamiliar country.
I have also seen people buying addon software so you can use it when flying, should you desire.
The ones with the cellphones would be handy if you really needed to keep in touch with e-mails away from wifi hotspots, but they are pretty expensive.
300-400 should be more then enough. I would reccomend Dell or HP. IMHO although I know a lot will bitterly disagree I would steer clear of Palms. They have a lot less software available for them compared to windows platforms. Its a bit like the Mac vs Windows debate!