If you are only going to use your mp3 player at the gym, then I would get the cheapest thing you like. That's what I did (I got a 512mb SanDisk item), cost about $80. It's flash based, so it can take getting dropped and whatnot, and it's cheap enough that if I break it I don't mind too much having to replace it (that would be my main issue with a regular 30 or 80 GB iPod-the hard drive is sensitive to shocks, so I would be loath to use it while running).
Here's the thing, though. I initially bought the SanDisk to use when running, just like you. Then I left it in my coat pocket and fished it out during a lunch break and realized I rather liked being able to listen to music of my own choosing while sitting in the breakroom. Then I realized I could hook it up to the portable CD/tape player in my office, so I didn't have to remember to bring CDs with me to work every day. Then I got a widget to plug into the tape deck of my car. Then I filled up the SanDisk and realized I really wanted more room, because sometimes you really want to hear Ogden's Nut Gone Flake and your copy is still at home. Which is how I ended up with a 60GB iPod that has several thousand songs on it.
If the above scenario sounds like something that may happen to you, then I would get something like an iPod Nano (or similar). No moving parts, but several GB of room so you can have plenty of music on it and not always have to listen to your "workout mix" when you decide to bust out the mp3 player on that long plane trip or during that fifteen minute coffee break where you just want to read and not be disturbed.
I would also stay away from those devices that have mp3 players, cameras, voice memo recorders, phones, tweezers, and other stuff in them. I've tried several of these, and while I think the convenience might be nice sometimes, they never seem to do any one of those tasks very well, and certainly not as well as a dedicated mp3 player, camera, etc.
Just the opinions of someone who has been there and bought that.
(The SanDisk is still going strong, by the by, although sometimes I can't get a signal out of it without twisting the headphone plug around a lot. I think the sweat has gotten to it).
-Wm.