This is a bit late too, but if you're still interested:
I have used a Yamaha 7.1 AV receiver (something like the RX-V661) using my MacBook Pro. You can then hook up the AV Receiver to whatever sort of speakers you like (Infinity, Boston Acoustics, etc.). Eventually, you can add (or connect an existing) HDTV and basically run a whole home theatre system from the computer through the receiver. Costs can be pretty high though, a good receiver is $300+ and speakers range from the sub-100 dollar range to thousands of dollars. This is my favorite setup, and it sounds simply amazing.
Due to space constraints (living in a dormitory), I don't use the receiver solution right now. I had looked for an inexpensive DAC to use with an amplifier that doesn't have optical inputs (I have a set of speakers that have a built in amplifier) but they are hard to find, or expensive.
I currently use a Miglia HarmonyAudio FireWire 400 soundcard. It sounds much better than the built in analog output and offers 8 channels of output (up to 7.1 surround). Coupled with a Logitech z5300e 5.1 system, I have a pretty nice surround sound setup. The HarmonyAudio can be had for 60 bucks, while the z5300e set is usually less than 120 these days, so it's inexpensive as well.
The HarmonyAudio could easliy connect a MBP to a regular receiver without optical inputs as well.
I would suggest going with a Yamaha or Denon AV receiver with optical input supporting at least 6.1, if not 7.1, as well as supporting at least 1080i. You'll have a quality investment, you can start with just a pair of speakers and add and improve as you like.