The 5.1 speaker system has plenty of room to expand.
It can be difficult trying to decide on individual speakers when buying separates. You need to make sure your center channel isn't too weak and that your surrounds aren't too strong, and after a while it gets confusing.
The latest offering from Phase Technology is a full speaker system designed to play nice. You can use the speakers with whatever your current receiver is, assuming it's not pushing over 100 watts per channel.
The system comes with four satellite speakers, one center channel and an 8 inch subwoofer, and can be had in a glossy piano black finish at a price of $938.
If you need to expand from there, you can pick up an extra pair of the two-way satellite speakers for $163.