Welcome to Audioholics, kman17
In your position I would get the 516, it doesn't have all the nice features the 816 does such as preouts (for adding a more powerfull external amplifier) and more inputs. The power difference is neglegible, probably not even noticible. The 516 is an inexpensive solution to your problem. The speakers you have chosen will work fine for that receiver. I have oned a pair of the 21's, they are decent for the money, they don't offer much bass, but you say you have the sub that came with a HTIAB. What is the make and model of the HTIAB?
It is nice to see other younger people interested in the world of audio.
When I was still teenager I was just getting into audio pretty heavy actually.
Good luck.