harman kardon soundsticks are just a type of speaker

you dont actually need an external soundcard for your macbook, you can just plug speakers into the headphone port and the sound will be great (depending on the speakers of course)
I have a set of Bose Companion III's that set me back £200/$315. There are obviously lots of choices when it comes to speakers and setups, all depends on what sound you like
As for an 'external soundcard' for your macbook there is the 'Griffin iMic'
http://www.griffintechnology.com/products/imic but unless you really want an external soundcard it's not really worth it, I had one for a while on my MacPro but it was just using up a USB port. The one advantage it does give you is that you have a bit more flexibility in the jack connection - if i knock the line out port on my mac pro i feel like im going to break something, with the iMic it just moves, so there's less fear of something breaking