I'm with the gang here on expensive cables as that's just snake oil. In almost all cases I'd stay away from power conditioners as well. Once you have everything paid for in your system, and still want to doll it up just for grins, because you got money to burn, then try them out.
As for separates, the sensitivity of your B&W 683 S2s are 89dB spl (2.83V, 1m); while the center HTM61 S2 is 88dB spl (2.83V, 1m). For illustration, assuming the latter as your average, and say you are listening 3 meters (10 feet) away from them (with no contribution from your rears) with an amplifier capable of 100 Watts RMS per channel, the SPL can be in excess of 103 dB. Hearing loss can begin to occur after only 7.5 minutes of exposure at these levels. (Even using 50 Watts RMS per channel is over 100 dB. Loud enough to begin causing permanent damage after just 15 minutes per day!)
So while I do recommend reserve capacity in amplifiers, jumping into a Ferrari to go across the street is a little excessive. I'd bet you can do just fine with a decent AVR. Of course, like jumping into a Ferrari is really cool, going for separates is often more about the "bling" factor than a real technical need.
Just my 2 bits...