Phenom II 955 has a thermal power design of 125W and it does get quite hot at full load, which of course is quite rarely.
What you want is a cooler that has:
- tower type cooler
- airflow parallel to motherboard
- fan placement freedom on the bottom side of the tower
- silent at low airflows
- low profile
Everything except the last one is easily done and there are several good contenders: Scythe Mugen 2, Prolimatech Megahalems (w/ optional AM3 retention kit), etc. Many tested here at SPCR in the reviews section. These paired with a good fan should be able to cool down your CPU very quietly.
You can find the Mugen 2 and others ranked alongside each other here:
http://www.silentpcreview.com/article961-page6.html
Is it low profile? Well, the heatsink is 30mm taller than the fan. If you want the kind of air flow direction as you explain above, then a tower cooler is probably your only option. That means a height of 125mm at the very least. Most tower are in the 150mm range, as is Mugen2.
So, they are not as low profile as top-down blowing heatsinks, but that air flow direction is against your requirement.
If you want a top-down cooler and don't care about the air flow direction being everywhere instead of towards the top of the case, then see the following thread.
viewtopic.php?t=54682
It lists current top top-down coolers, most of which are also available with AM3 retention kits.