If you are still checking your post after two days with no response, I'll offer my thoughts to your question. In my opinion, there are really only two reasons to replace the stock cooler / fan assembly.
Reason 1: You are going to overclock and need the extra cooling offered by an aftermarket heatsink / fan.
Reason 2: The stock fan is just too noisy. Before you replace the stock HSF due to noise though, try to enable any CPU fan speed control you may have in your bios to a lower setting. This can often bring noise levels down to acceptable levels. However, make sure you also keep an eye on your CPU temperature to verify that any lower fan speed is not resulting in unacceptably high temperatures.
Personally, I have always had good performance in both cooling and in noise levels when using the stock cooler supplied with AMD CPU's.
As for the thermal paste, the pre-applied paste from the manufacturer will really be sufficient. There is no reason to add more paste if the heatsink already has a layer on it.
Let us know how you did with your first CPU installation. Did you find it went together easily or was it more difficult than you thought? Best of luck.