There are many different configs - from simplest to...more expensive:
1. iMac headphone output direct to stereo powered speakers / active monitors with analogue inputs (inputs 3.5mm, RCAs or XLRs). e.g. JBL LSR305 around $200 or Focal Shape 65s for $2,000)
2. iMac USB output to stereo powered speakers with USB inputs (e.g. Audioengine A2+ $250) - this isn't tons different from 1 except you're now relying on the quality of the speakers' DAC being better than that of the iMAC)
3. mish-mash of 1 and 2 - e.g. Dynaudio Xeo 2 ($999) - these speakers can be run from analogue, optical (which wouldn't work from the iMAC without a USB-optical adapter), and Bluetooth wireless
4. iMAC USB out > external USB DAC > stereo powered speakers. You're relying on the quality bump of the external DAC here. e.g. Chord Mojo ($500), Chord Qutest ($1800)
5. iMAC USB out > USB amp > passive speakers. There are now a neat range of amps with USB inputs. See here
http://www.the-ear.net/review-hardware/desktop-usb-amplifiers-usb-amplifiers -- you're reliant on the quality of the amps' DAC, but have more choice over which passive speakers to go with. Cost variables are which amp, which speakers. Speakers could be anything from say $100 to let's say $1,700 for hmm, ATC SCM12 Passive Pros
6. iMac USB out > external USB DAC > conventional analogue stereo power amp > stereo pair passive speakers (this would just be like a normal home hifi setup with computer as source). Could run from a couple hundred bucks to infinity.
I think a sane person would choose 1. An audio nut would go 4 or 6