Good DAC for Macbook Pro and Infinity Creshendos

I know very little about computer audio; I have a Squeezebox that I use to stream CD's ripped to Apple Lossless to a Fisher tube integraded amp.

I am working out of town a lot and have an apartment; what I would like to do is find something that requires little monetary investment to get me going. So, I have my Fisher X100, my Macbook Pro, and external hard drive, and my Infinity Creshendo 3006 bookshelf speakers. What can anyone tell me in terms of products to look at? I will be playing FLAC in addition to Apple Lossless. Thanks!