If you are seeking to utilize a dedicated amp, I would look at separates. If you feel that's too pricey, you could always buy an entry level surround receiver off the shelf (Onkyo, Sony, etc.) and sell it later on.
What do I look for in a surround sound receiver so that I can add an AMP in the future. I have a Klipsch 52 center (with a pair of matching floors coming soon.) and it seems few receivers (with internal amps) have the juice these speakers want.