HIGH END IDEA:
Pick your choice for a USB DAC/Preamp, such one of the following:
Used or "open box" Focusrite Clarett 2Pre on ebay for $285-300. This is one of the best sounding pro audio interfaces for the money. Company provides software drivers for Mac.
or
Audio-GD NFB-11.28 for $339-389 (depending on model chosen). Excellent audiophile dac/pre. NO Drivers needed for Mac OSX 10.6 (or higher).
Then buy a really nice set of powered studio monitors such as Yamaha HS-5 ($399 pair) or similar. If you are willing to spend $800 and have the space, the Yamaha HS-8 are excellent and have a ton of bass.
LOW END IDEA:
If you want everything built into the powered monitors, including USB input, then you are pretty much limited to computer speakers such as Edifier, Bose or Logitech. Edifier has their s3000pro for $800, but at this price you are going to get better results from the DAC/pre + Yamaha above. The Edifier sounds a little flat/sterile in comparison; the Yamaha will end up being more "real" and natural sounding as well as more open sounding. Check out Best Buy website or New Egg.