You'll need some sort of amplification between passive speakers and your source. Usually this takes the form of a receiver.
There are powered speakers, but generally the only cheap ones are in the form of computer speakers. With powered speakers, bass management can get tricky since you wouldn't have a receiver in the path to handle it.
If you were to start with a 2.0 system with the upgrade path toward a 2.1 system (and then possibly eventually a 5.1 system with a receiver), you'd be able to get a simple amplifier to go with your first pair of speakers and replace it later.
For example, if you got a t-amp from amazon
http://www.amazon.com/PORTABLE...&qid=1225257919&sr=8-1
and then got a pair of inexpensive bookshelf speakers somewhere else (I have a list of cheap ones in the sticky thread) or used ones...
that would get you a decent music system going
I don't think it's feasible to do 5.1 on that kind of cash though. A receiver is really the best option in most cases for doing what you need.