Let's just clarify some terminology first. A PowerBook doesn't have a "sound card" per se. It has built-in audio hardware on the logic board that powers the headphone/speaker output and the audio-in mini jack. The built-in hardware is limited to 16-bit, and the signal-to-noise ratio is not all that great. And 1/8" minijacks aren't the cleanest for I/O, either.
Your best bet is a FireWire or USB audio interface. Under OS X, you shouldn't have many problems with latency... OS X is a leader with its USB audio capabilities. M-Audio has a good range of products for USB audio and MIDI. I believe they also have a FireWire interface coming out in the next couple of months. Edirol (a division of Roland) also has some products to consider, as does Emagic. I'd tell you specifically which ones i've been looking at, but their site is down at the moment.
Make a list of what you need (1/4" vs. XLR input, S/PDIF digital I/O, MIDI, etc.) and figure out which interfaces will meet your needs, and whether they're compatible with the software you plan on using.