I dont think that you *really* need to change them, especially if you are happy with the noise levels and temperatures in your case.
I am no expert in this area, but from reading this thread and others in here, is that replacing the stock fans is to do with moving more cool air for the same speed or less speed, (which makes things quieter), everyone seems to like different types of fans, I chose the Sharkoons just because they are white....
I got the faster ones so I could slow them down and get a balance of noise/airflow.
Whether you need the extra fan in your case or not is again down to whether you feel that your temps are good or not, if you want to attach the front fan, looking at your setup you would need to move your IDE HDD out of the way, which is gonna cause problems connecting your HDD and CD drive to the one port.