Well, for under $80, you could get the M-Audio Transit, which is very small and portable, but leaves you with 1/8" mini jack connections, if that bothers you. But you also get a digital optical output. In truth, the Creative Labs Audigy 2 External, while made more with the gamer in mind, is not bad, either. You could get the MBox to work, and it would sound great, it just wouldn't be as intuitive. I have a Digi 002 system, and it kicks, but Pro Tools LE can be a bit system finnickey. The Mackie Spike or the M-Audio USB Audiophile DAC recommended above could work, too.
Tom.