If your study guide is 128 mb you would need to have it on a external memory card ie SD, XD, (whichever type your pda uses). Most internal memory for PDA's only goes up to 128 mb's.
As far as your Davis drug guide...it has to be in a usable format for you PDA. If it says it is for a PDA than it will work but if it is just the disc out of the book it will not as those are usually just for PC (your home computer). Also another thing to realize is there are two different operating systems commonly used in PDA's one is Palm OS and the other is Windows (the mobile one don't know the name off hand). But programs are only good for one or the other, so it is a good idea to wait to purchase software until you have your PDA in hand unless you know what operating system you are getting.
Another great program to have is a Webster's dictionary. I have looked in to this and purchased the actual Webster brand as it is the most comprehensive and it also has a thesaurus. But this is a very large program (like your study guide) so it comes on a external memory card...I just leave it in my PDA that way I can always check my spelling
while charting etc (I'm terrible speller he he).
I hope this helps a little good luck all,