I think youll love the px100 with an ipod.
I use mine with my Zune player,and i like the warmth of them together.
The Ipod has a little more forward midrange than a Zune,and with the laid back mids of the px100's, i think it would balance out nicely.
I think the Ipod can also use the extra bass the px100 will provide.
For the price range,the only other headphone i might suggest is maybe Igrados,or Portapros(or any of the 60 ohm Koss models), but thats only if you like sparkly highs of Koss or Grado.
So id say,go for the px100 for the Ipod.
An amp would be nice also with it though.
Im looking at the Corda MOVE for my setup.
The Corda lll is also good from what i hear,if you dont wanna spend as much.
Ive also looked at the C&C box + as an inexpensive model.
Theres so many models out there. Look at SkyLabs reviews of his 20+ portable amps on this forum.