Advice on Setting Up A Laptop-Based Audio System?

Hey, I'm looking to set up a workspace audio system based around my Apple Powerbook G4. I've got about 100 GB of uncompressed audio on an external hard drive. I currently have Monsoon planar speakers and matching subwoofer hooked into the laptop through the 1/8" audio out, which soundsokay, but I think I could do better.

I don't have a PC card slot in the Powerbook, so I'm limited to a USB audio connection. I'm not looking to hook this system up to my "big rig" -- it's just for my workspace. I was looking to bypass getting an amplifier, unless it's fairly small.

I was thinking of going with some powered mini-monitors and maybe some kind of adapter/DAC in between, but I'm space-constrained and not looking to spend more than say, $600 total.

Any suggestions?