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You won't need a separate IDE because smalltalks usually come with their own IDE, so choosing your smalltalk flavour pretty much determines the IDE for you. Don't let this fact scare you off from taking on smalltalk though!
WRT your original question, I had two wonderful years developing in Dolphin Smalltalk & highly recommend it.
Dolphin Smalltalk is only as free as a beer is though. If you need an opensource smalltalk go with Squeak.
In my opinion Dolphin is the more polished/comfortable/user-friendly one.