I was exactly in your position last year, and after some research I went with Unbound over Skyscape. I liked Skyscape's Constellation bundle a little better over Unbound's Nursing Central, BUT after careful reading of fine print, Skyscape requires that you repurchase the bundle every year in order to even have access to the texts! This is only true for Apple products with Skyscape, not with Smartphones and PDAs. Other companies like Unbound require a yearly subscription in order to stay current and to have access to Medline and such through them, but once you purchase the digital texts, they are yours for life.
Skyscape may have changed its policy since last year, so check again. If they have changed it (and you really have to read carefully to find this clause) then I'd say to go with Constellation, as it has RNotes in it as well.
That said, I rarely used the Diseases text that came with the package--it never seemed to have what I was looking for. I did use the Diagnostics text sometimes, but not regularly. What I DID use the living heck out of was the Davis' Drug Guide and the Taber's Medical Dictionary. I honestly think that you'd do well with just those two to start out with, plus the free apps. Unbound has a $1 app for subscribers called Diagnosaurus which is pretty handy, and Epocrates has a free "lite" version that has a nice visual drug guide, in case you have a pill in front of you and no idea what it is. I need to use Epocrates more, it has some neat features that I'm just not that familiar with yet.
As I advance forward, I'm looking at adding Harrison's and the 5 Minute Clinical Consult, but those aren't things that you'll need at this point.
Take a slow trip through the medical section of the App store, you'll find lots of cheap or free apps that are useful. Some of them, like Skyscape or Unbound, say free but really require the subscription through the company. Others are simply third-party apps that are low cost. OB wheels, vaccination checklists, various medical calculations--you'll find a ton of things there.
I don't know what I'd do without my iPod Touch on the floor (or in class)--I find that I learn and retain even more because if I run across something that is unfamiliar, I can look it up immediately. Fellow students come to me now and ask me to look things up for them, too. I also like the Touch over the iPhone because some institutions won't allow camera phones on the floor, and most places require that phones be turned off.
Aside from apps, there are tons of free nursing podcasts, nursing and science course lectures, and nursing videos available at the iTunes store. Check both Podcasts and iTunesU. I literally have hundreds of hours of nursing audio on mine, I listen while I walk the dog, clean house, etc. It's a great way to learn new things and reinforce what you've already learned.
The Touch is incredibly handy, you're going to love it! All this nursing goodness PLUS music, games, video, internet...I think everyone should have one of these, they're amazing. Enjoy!