When I bought my first PDA for nursing, the AACN website had some good PDA offers.
You could buy a PDA with bundled software. You would get a PDA that was shipped with a handful of programs geared toward nursing. Everything from a drugbook, references, calculators, etc.
Here's the link, take a look:
http://www.aacn.org/AACN/conteduc.nsf/vwdoc/PDASpecials
Best of luck.