AudioEngine 2, $199 (or less if you find an on-line coupon).  Better and cleaner sound than the Bose or others, and MUCH better quality construction, too, they'll last.  And they look really good with a Mac.

http://audioengineusa.com/Store/Audioengine-2

If you feel adventurous, and have more time than money, seek out a pair of good-condition Realistic Minimus 7 speakers. (eBay; don't spend more than $50). Black were the most common but they were also available in white or silver, the latter would be a great visual match with your new Mac. Unlike most other cheap speakers, they are also repairable without a lot of effort or money. Nice utility speakers, useful in so many ways, and you will be pleasantly surprised with their sound. With the money left over, go further retro and power the Minimus 7s with an old, small stereo receiver or integrated amp.

I run just such a setup for my garage system, although my Mac isn't in the garage. I use an AirPort Express plugged right into the back of the receiver's unswitched outlet.