I recently built a system with the Phenom II 965. I used the stock heatsink at first. If low noise if important to you, I highly recommend you go aftermarket. The stock cpu fan reaches speeds in excess of 6000 rpm, which sounds a bit like a drill.
I replaced the stock HS with a Scythe Mugen 2, which has a fan that never exceeds about 1200 rpm, and is quieter than my case fans. Plus, my cpu temps have dropped and I have overclocking headroom.
Noise is an issue for some and not for others. It is much easier to start out with an aftermarket heatsink during your build than replacing the stock one later.