That Delta is a dual blade design and the noise level is huge due to the airflow between the two sets of blades. Yes it move a ton of air but it is not a great fan. Check out the air presure to air flow chart found <A HREF="http://www.delta.com.tw/products/dcfans/pdf/FFBseries.p..." target="_new">here</A>. (Look under the 80mm fans)
See the huge kink in the line. When looking for a fan you want a straight line across is closest to the top right hand corner.
There are a lot of good fans at 80mm.
As for 80mm fans, yours is rated at 52.5dBA at 5700RPM. A rheostat could help control the speed but if you look at that graph, when you decrease the power the line shift down and to the left. (AKA one line down.) On a fan like that you loose a lot of CFM/mm^3 when you decrease the power.
However, you might want to get the 92mm conversion pins and go with 92mm instead of 80mm.
92mm fans can get you even more airflow at a lower dBA.
Is there an airflow that you are looking for? What kind of heat disipation are you looking for? In your current configuration you should get about 0.16 C/W to 0.20 C/W depeding on the processor. Putting a lower CFM fan will raise those figures and the resulting CPU temperatures.
What CPU model is it for? What speed are you running that Athlon at?
Got a LAN Party that you want people to know about?