Looking at the AM3 possibilities the review poses at the end, I see that the Xigmatek HDT-SD964 is no longer available. The closest seems to be its dual-compatible SD963. Anyone know if it's the equivalent for these purposes? Thanks,
Afterthought: The potential trade off of another CPU fan is that this case comes with a 3" (7cm) side panel air duct directly over the CPU. It is easily removable, but seems like a handy ventilation feature for a fan that faces upward.
That SD964 is quite an old heatsink: moreover, I don't think it would easily fit in your actual setup (166mm - 70mm = 96mm, if I'm not wrong).
If you want to go with a low profile cooler, maybe the finest available right out-of-the-box for a silencer could be the Scythe Big Shuriken/Shuriken 2. Well, there's also the Thermalright AXP140 (95mm or so about) which according to me is substantially better, but usually it's kinda pricey: so it's up to you. If you wanna buy another fan, there are a few more, like the now EOL CoolerMaster GeminII S, or the Prolimatech Samuel 17.
But I would stress that quietness is more than picking some parts, it's most an operating procedure.
Just for example, IME any 120mm running above the 800rpm mark is noticeable, and at 1500rpm it is loud, so you have to calibrate its speed using a fan controller (like a Scythe Kaze Server), or a software (like SpeedFan, or maybe your EasyTune) accordingly to your thermal needs.
Whether or not the stock heatsink and fan may be good enough is IMO mainly a matter of personal tastes and expectations.
In my experience, stock heatsinks are just crap (both Intel and AMD). Even if sometimes it can be conveniently dialed down to an acceptable noise level.
Eventually, about the duct, it's an outdated thermal solution, IME of dubious effectiveness (at least with aftermarket heatsinks).
If I were you, probably I would get rid of it and choose some relatively cheap but good tower heatsink, like a Cooler Master or a Xigmatek, swap their horrible sounding stock fans, and then pick something like a lower speed Scythe PWM.
If you still want a duct, you can still duct the heatsink to the back-panel fan.
Hope this helps.