You have A LOT of dust and grime in there...you should start by vacuuming out the dust!
(you should vacuum the dust out of the radiator too - after you take off the fan).
Does this look like your cooler:
http://www.frostytech.com/articleview.cfm?articleID=2699
1. How are the processor temperatures under full load? (If they are low enough, you may just be able to reduce the fan speed.)
2. Are the fan cables connected to a motherboard CPU fan header or to the cooling pump block?
3. Is there a cable that goes from the cooling pump block to the motherboard CPU fan header, or is it connected directly to the power supply?
4. From the model number on the radiator fan, it looks like it's a 120mm Fan. Can you measure the length/width of the fan on the radiator?
5. What sort of price range are you looking at for fans?
The radiator in there is pretty thick, so you can reduce your noise by running 2 fans on either side of the radiator in push/pull (if there is room to move the cooler 1 fan thickness (25 mm) farther into the case.) With 2 fans like this, you can lower the RPM on both to get better airflow.
What sort of fans are in the front of the case? The large dust build up points to either no fan filters on the intake, or negative pressure (more fans blowing out than blowing in).