If you start doing clinical work in years 1-2 it might make sense to get one then. I know we did a fair amount of clinical experiences in year 2 for which I found my PDA useful. And I loved my Tungsten C's wifi while sitting in class. I also think a decent drug reference can be useful in years 1-2. And if you need a good organizer, Palms and PPCs do a nice job for many people.
Right now I recommend Palms for most people (I have both). I think an expansion card is a must. For entry level and basic stuff, I think the E2 does a good job. The E2 is $200 new, and $160 open box at Overstock. If you need a keyboard, Wifi, and a good processor, a Tungsten C is good. If you don't want a keyboard in favor of more screen real estate, a TX is nice. If you think you need a phone/PDA combo (I've avoided them due to concerns about getting yelled at by attendings who think I've got a phone on even though I've turned the phone part off) a Treo 650 is nice. I love my new Dell Axim x51v, but so far I think a Palm will suit most basic medical needs better - simpler, more free software, better battery life (I think), and it seems quicker to do things on it. On the other hand, if you want a PDA that can stream MP3s over Wifi, display the current weather on your Today screen, play 3D games, and run some medical programs all at the same time, then PPCs are great.