There is an entire forum created just for this topic http://allnurses.com/mobile-computing-nursing/
Your basic choices are first do you want a dedicated PDA or a PDA combined with a cell phone
For a stand alone PDA you have your choice between Windows Mobil and Palm operating systems
Despite what actual device you get you still need to buy software.
Since your school is requiring a PDA I would contact them to make sure what every you do buy is compatible with the software they want you to have.
You can get a free up-to-date drug guide from Epocrates.
The most populat software is Skyscape. They have all the basics from Davis Drug Guide to Tabers.
Check out the forum for more details on specific devices and software programs, just about everything out there is covered in the forum.