The Treo line of smart phones are best.
Even the Treo 600 works well as a PDA, cell phone, and MP3 player. If that's all you are looking for, you could buy a used 600. That's what I'm using.
On Friday, I helped out a close friend (who had a nasty cold), by strapping her baby on my back, walking her to sleep, and carrying her for a 2.5 hour nap. During that walk, I was able to use my Treo 600 to: 1) take a picture of my friend's baby sleeping; 2) surf the internet, check email, and do some stock trading; 3) call my friend and update her about her baby's nap; 4) jot down some notes for an article I'm working on; 5) play some tunes to help the baby sleep. Then, yesterday, I used the Treo 600 to caculate the times for the high and low tide, and the sunset, to plan some outdoor photography. I also use my phone for sunrises, moon phases, azimuths, addresses and phone numbers, depth of field calculations, storing and reading camera gear instruction manuals in PDF form, listening to audio books, reading text books, playing video games, and as a watch.
The 650 has a number or nice little improvements, such as easily replaceable batteries, a better illuminated keyboard, higher resolution screen, better quality camera, the ability to record videos, the ability to watch movies, the ability to make record audio, etc.
I'm not familiar enough with the 700, but I imagine it has more nice little improvements. I seem to remember it has faster internet.
Mike