2 ch. or 5.1 amp for HT mostly for music

I have just gotten back into audio after having raised 3 children. I bought an HK 325 receiver and 2 rti polk speakers along with a center. At some point for HT, I will probably add surrounds. Here's the problem. If I had it to do again, I probably would not buy any of the above, although neither the HK nor the polks are terrible. I listen mostly to music. Would I be better off to buy a 2 channel stereo amp or a 5.1 amp. I'm leaning toward a 2 channel. If I do go with the 2 ch. will this stll work when I add the surrounds. Again mostly music for me but some movies for the wife and family. I am also in the market for some good used speakers.