Stock fans in general are less than ideal but the ones for the AMD Phenom II CPUs are especially so.
I was afraid to use it because of the amount of tension that was being placed on the AM3 plastic bracket.
I would definitely go for an aftermarket fan.
I used this on a friend's AMD Build:http://www.newegg.com/Product/Product.aspx?Item=N82E16835103100
It worked out really good.
Just keep in mind that if you have ram with high heatsink fins you may have some clearance issues.
The Cooler Master EVO is another good one but it is a bit too large / heavy for my liking.